During the game's early development the human players never existed, and both players would spend the entire game in the Metal Slugs! This version was demoed in location testing and at the Amusement Machine Show 1995 (アミューズメントマシンショー, or JAMMA) from September 13 to 15. The image above is cropped from the show's brochure. (big ups to Video Game Densetsu for documenting this in the first place!)
White soldiers appear to be aiding the players against Morden, which backs up the reasoning behind the unused hostage soldier graphics as well. You can just about make out a different health bar in the top screenshot, which can also be found in the ROM. The Metal Slugs' vulcan shots appear to use the same tiny bullets as the turret, bullet casings are also being ejected from the vulcan, and the golden tank's shell is firing straight forward - did they have the Armor Piercer implemented at this point?
Left: On another part of the above flyer is art of a white soldier wielding a pipe bomb.
Right: Cropped from the cover of another flyer, this depicts a soldier riding in the Metal Slug. That'll be discussed further down the page; this image is probably the most accurate to its (then) in-game colours.
I'm going to foolishly try and list the early footage in order of release. Credit goes to Ivo for making most of these comparisons and observations!
Gamest Vol. 151
Dated September 30 1995 (though all Gamest were published a month before according to "Total Capture!! Gamest").
As far as I'm aware, this is the earliest version of the game shown publicly. It features a different story (expounded upon in Gamest #152), as well as different characters: Phil Gene (フィル・ジーン, alternately translated as Phil Jean) as player 1, a Regular Army mechanic, and Michiko Nakajima (ミチコ・ナカジマ), his girlfriend and assistant as player 2; they are depicted only in early concept art.
Even at this point in development six missions were proposed, though only three are shown in this version; Mission 1, Mission 5 and Mission 6.
This How To Play screenshot shows you how to follow branching paths by pushing diagonally on the stick; this feature exists in-game but is never demonstrated here. A white (friendly) soldier is present and presumably giving the instructions; those frames of animation are used for other purposes in-game, but are still located right alongside the How To Play graphics in the data.
According to the magazine's description of the controls, the C and D Buttons are totally unused. The A Button shoots and the B Button jumps, though the A Button will fire the cannon and vulcan simultaneously. The vulcan can be locked (Vulcan Fix) by holding the A Button, and it is aimed with the stick just like the final - this is changed in a later revision. It also makes a curious remark, "it is possible to fire different bullets while crouched" (このときに自機が伏せ動作中 であると通常とは異なる弾を 撃つことができる。); the only possible allusion to this is in the next issue below.
The original version of Mission 1, with a totally different background! A massive portion of this stage was redrawn entirely for the final game, though its original art was shown off in Retro Game Test's Metal Slug History video (see below). It hits all the same beats as the final, but takes place almost entirely in marsh and swamp. The level design has a lot more nooks and crannies for enemies to stand on or hang from.
A captured soldier is present, showing those unused frames were once put to use. You can see the white ally soldier has thrown a pipe bomb, while the enemy soldier is throwing a pineapple, a tradition maintained in the final (and also explaining those unused animations). Prisoner soldiers are coloured blue, but change colour when rescued - white for player 1, gold for player 2 - and assist the Slug. In this case, they roam alongside and automatically attack targets with grenades or knife attacks.
The Slug's shell has presumably exploded next to the dying soldier on the right; it's a lot smaller and subdued, using the same explosion sprite as the Rebels' landmines. The HUD is different, showing only the player's score and lives - it seems the Metal Slug had infinite cannon shells in this version, though it's unclear how much damage it could endure.
A Di-Kokka! They don't appear until Mission 2 in the final. Its projectiles are long and twisting shells instead of fireballs; those graphics are unused but can be found in close proximity to their sprites in the final.
Past the submarine and towards the village. A soldier has grabbed onto the Slug and killed its rider (shown in screenshots further down) in the first image; soldiers don't grab onto the tank until Mission 4 in the final.
Both players in action; player 1 is white/silver and player 2 is yellow/gold. A lot of the treehouses are structured differently compared to the final.
The first boss is sporting two mounted guns on its wing! These still exist as unused graphics in the final, though the left gun only has sprites for facing forward, it's unclear what projectiles it would fire that could have harmed the Slug. The Tatsuyuki's laser is yellow here and in all tank-only shots, but in the final game it is blue.
There's no box around the score tally.
Interestingly, the text says only two ally soldiers would be present at a time, but anymore you rescued would be kept as 'stock' on the bottom of the screen. When an ally dies, another soldier would enter the scene to replace them. Whatever stock you had at the end of the level would be added as bonus points.
An ally soldier is riding in the tank's turret; this is another way rescued soldiers can support the tank, by firing their bazooka every time you press the A Button, and also serving as a one-hit meat shield for the Slug. Most of the associated graphics were removed (rotations, etc), though it was repurposed for the Shoe & Karn boss in Mission 4. His muzzle flash and missile are not in the final (the boss version uses a different twin missile), though it looks a bit like the shell attached to the unused KT-21 tank.
The Slug's cannon shell is positioned lower than normal, and you can just about make out the bullet casings the turret is dropping; these appear consistent across all the tank-only version screenshots.
The Metal Slug's hatch is open to accomodate the soldier, which only happens in the final when crouching. The hatch uses a different sprite than the final, shown from a different angle...
... though the graphic used when the soldiers lob bombs in the hatch do look similar. Thanks to Ivo for making the comparison!
The Iron Nokana's front plate is missing its spikes and grey colouration.
A friendly soldier manning the turret! An ally soldier would automatically hop into one and fire at enemies, according to the magazine, whether they're on-foot or in the tank.
The turret appears to be asymmetrical in the second image by missing its left ammo box; it may just be a fluke, but the bullet casings don't appear to be rotating either. This also best shows that the Slug's vulcan fired the same type of bullets as the mounted turret; the final get them beefier-looking blue projectiles.
A good look at the Slug's early vulcan shots, shell explosion and bullet casings. The destroyed Slug in the background isn't visible, though it may just be covered up.
Neo Geo Tsushin Vol. 2
Dated September 15 1995. images from Brian Hargrove's Twitter.
Still no bounding box on the score tally.
A very different rendition of Mission 3; this was another level almost complete redrawn for the final version, and its art was shown off in Retro Game Test's history video.
Either there's a second Iron Iso off-screen, or its turret has popped off similar to how a Girida-O is destroyed.
Gamest Vol. 152
Dated October 15 1995. This version changes the control layout, but otherwise looks much the same as the last issue's coverage. The release date is unknown as of its writing. It also provides a more in-depth look at the story than the last issue:
In 20XX, General Morden enacted a military coup he had been planning for years in a bid for world domination. With coordinated support across the globe, Morden and his Rebel Army conquered the major cities of the world in just 170 hours. The Regular Army was crushed in a number of surprise attacks, though the survivors formed a resistance movement to push back against Morden's regime.
Enter Phil Gene. Before Morden's takeover, he was a developer in the Regular Army and was rushing to complete the new all-purpose tank (Super Vehicle) to be at the forefront of their next campaigns: the Metal Slug. However, a Rebel attack force had finally pinpointed the location of a secret factory belonging to the resistance. Lacking the man and firepower to repel them, the factory was reduced to rubble.
From behind the debris, another tank appeared. With one blast of its cannon, the Rebel tank "Di-Cokka" was blown apart. Phil's test run on the Metal Slug had begun. On board the prototype SV-001 was Phil, with his talented assistant and beloved lover Michiko Nakajima in the SV-002.
Now, the two-tank army began their march...
The How To Play screen's instructions tell you to push Up to jump in this version.
According to the magazine's text, this version redid the entire control scheme; you no longer aimed the vulcan with the stick, but instead each of the four buttons directed it a certain direction: A Button would fire forward, B Button upward, C Button backward, and D button downward. Any button press would fire all weapons at once.
Holding multiple buttons at once would allow you to aim diagonally. You can identify any screenshot with this control layout by checking if the vulcan is only ever facing straight or diagonally, never in-between. The magazine politely claims "it takes some time to get used to," but remarks it may be changed in future revisions.
The early entrance to Mission 1's village. You can see how the Slug's cannon, vulcan and the bazooka soldier fire simultaneously. The Slug's jumping animation looks identical to its idle stance in these shots.
Despite the different background, the village remains similar to the final, though the finished game has a platform where the Slug is to let players jump up on the structures.
The mounted guns are still in place; are those little yellow streaks by the top-right door in the second image its projectiles? A strange black ball is visible on the ground, and the second shot shows the Slug seemingly firing its cannon while crouching; you can only throw grenades while crouching in the final. This appears to be evidence of the Slug firing "different bullets" while crouching mentioned in the previous issue.
A crouching rebel in the balcony; it's all bazooka soldiers up there in the final. There appears to be 4 birds in the bottom corner, but only 3 in the final (OH BOY!)
Two dogs, including one sniffing (using an unused animation).
The usual assortment of Slug changes; bullet casings, bazooka rider, different vulcan bullets.
The boss is still missing its grey spiked plate.
Final mission with friendly soldier.
The turret is still missing its second box, and the casings are all at the same angle.
Gamest Vol. 154
Dated October 30 1995. Besides updating the end-of-level score tally, this versions seems much the same as the previous issue's, complete with the strange button-based aiming scheme.
From the looks of things, a friendly soldier has mounted the boss's gun and fired it against itself! What a versatile little bugger!
They finally incorporated the box around the prisoner tally!
From a slideshow showcasing the enemies that climb aboard and hurl grenades inside the Slug. The Slug appears to be totally replaced with an explosion sprite during the impact; not sure if that's an error or the magazine switched the order to make it more clear what was happening.
More bazooka rider action.
A soldier appears to be crawling out of a destroyed APC, something that doesn't happen in this mission. Also, where did all those grenades come from?!
More of the same Slug differences, plus a fleeing prisoner soldier.
A falling prisoner soldier.
Same turret quirks as last time.
A rare demonstration of the soldier-on-soldier combat, with a friendly knifing an enemy.
Neo Geo Tsushin Vol. 3
Dated January 1996. This issue has no actual coverage, it only appears in a "coming soon" column. No release date, but it's already marked down for both a cartridge and CD release.
The usual differences relating to the Slug and its rider. Notably, this is the only screenshot so far to show enemy landmines and a Melty Honey, albeit destroyed. They don't appear in Mission 5 in the final, but they do appear there in the bad ending.
From this point on the exact sources of the images are unknown.
Images ganked from Imgur, probably from Neo Geo Freak? It appears to be the same revision shown in Gamest #152, going by the How To Play screen telling you to press Up to jump. Yuck. There's a sniffing dog in that right image, among the other trademark changes.
The boss is still missing the spikes on its front plate. The right image has the usual changes; the turret's bullet casings, the vulcan's muzzle flash, and the Slug's cannon smoke should still be visible.
An early screenshot highlighted in Retro Game Test's Metal Slug History video (where did they get it from?). Just to show yet again those tied-up soldiers were used at one point in the game's development.
The Slug's shell is positioned lower than normal, and that orange burst does not exist in the final. The Slug's cannon explosion can be seen above the grate. The awning on the far-right appears to damaged, but the building untouched; it only appears damaged in the final when the rest of the frontage is wrecked too.
This image (also from Retro Game Test) shows off the original version of Mission 1. Captured soldiers, whole new backdrop, pipebomb-wielding allies. The usual!
Not exactly early footage, more concept art, but what the heck: Mission 1's original level art, shown in the Retro Game Test video. There's a few minor differences (probably cropped out) compared to the footage we've seen, like the absense of a tiny mound between the submarine and the village (see above).
And the original art for Mission 3; due to the lack of coverage in magazines, I'm not 100% on how it would flow. The Rebel base has a ladder the final is missing, but aside from the stone staircase, most of it is totally different art from the final.
Some of its assets popped up in strange places, though: the railing on the broken bridge (left image) was reused in Gerhardt City for both good and bad endings... and a portion of the background behind the bridge resurfaced in Metal Slug Attack, of all places!
Also ganked from Imgur, probably a later issue of Neo Geo Freak, but most definitely a later revision. The HUD now features a lifebar with five pips; it would be redesigned for the final and scaled down from five to three hits; this old lifebar is still present in the graphics data though.
Mission 1 features an Iron Iso where one does not appear. There's more projectiles on-screen, including a homing missile and shots that look to be passing through the Slug. The soldier is using the unused pipebomb animation, and behind him is another soldier in the middle of an unknown animation. The tower behind the Slug has decorative ladders, possibly for enemy and ally soldiers to climb up, while the final has platforms to jump on.
The area shown here is designed very differently from the final version of Mission 3. It might be down to the image's hue, but the Iron Iso in the left image appears to have more red markings; the sandbags on the bottom are not seen in the final, but exist as an unused item in a test stage.
This came from an arcade flyer for the game. Enomosiki pointed out the ammunition counter is "000", when it should be an infinity symbol. In-game there's normally a soldier and an explosive barrel where the box is, though there could be any number of reasons why they're not present.
Rage Quitter 87 supplies this scan from the Saturn port's manual (though it also appears in an arcade flyer), featuring a larger amount of paratroopers than usual and, strangely, two aquatic mines instead of an electricity pylon; however, there seems to be electricity coming out of them, featuring a different stream than the pylons, which is odd. That prisoner may or may not exist there in the game, what with the highly randomised prisoner location patterns this instalment has.
More Saturn scans that feature 000 ammunition, and amusingly enough, the middle two screenshots have 1P as the life value instead of the usual 1UP. Wacky!