Prototype

Video game development preservation group Hidden Palace released a prototype version of Metal Slug Advance in June 2020. No details are given on its origins or whereabouts, but I would speculate it might be the "new build" shown off at Tokyo Game Show 2004:
It resembles the final product in gameplay and content, but is lacking a lot of functionality for things like items, menus, and even bosses. That and its lack of backup battery, allowing players to choose any stage they please, suggests it was made for ease of access at trade shows.

Its sprite data is largely identical to the released version, but almost all of its background graphics are subtly different, and were overhauled for the final product. Even when level layouts are the same, background details are typically more fleshed-out in the final release. It's likely all the content had been made for the previous build of the game, and this is them still in the process of recoding everything.

Prototype imagery on this page will be displayed on top or on the left, and imagery from the final will appear below or on the right.
To help explore this stuff for yourself, use the savestates and Cheat Engine tables courtesy of Mr_Beacon00 (信标先生); the "no-pointer" table requires you find an address yourself (preferably something easy to find and modify like the bomb counter) and choose "Recalculate new address", while the others... I never got working. It's intended for VBA-M 32-Bit, but after that you're on your own!

| menus | gameplay | Missions | cards | object IDs |

Menus

I'm told there's a RAM address that will toggle the menus to English, but I've yet to get it working. The text is visible in the ROM though, and will be referred to when it differs from the final product.

Pressing any button during the SNK Playmore screen will skip to the title screen.
The prototype's logo was partially redrawn: the "E" in Metal and "VA" in Advance are slightly broader, along with changes to the drop shadow and backdrop transparency in the top part.

The prototype does not have a functional file select or battery backup yet: selecting any file will take you to the character select no matter what, and then to Base Camp, with no difficulty select or intro cutscene.
The auto-fire option is not available yet, but can be toggled on somewhere in RAM.
The save files are hard-coded -- the game does not write a save file to battery, and erasing and copying them is just for show, and will reset the moment you return to this menu. Some of the English text strings are formatted differently than in the retail game.

The palette on the character select screen is different; palettes across the board are a smidge darker in the prototype, typically on backgrounds. Walter's portrait isn't fully cleaned up, sporting aliasing on its sides, while Tyra's unselected portrait actually has more colour definition on her hair than the final.

A lot of the functionality in Base Camp has yet to be properly implemented.
The 'translucency' sprites used for the Base Camp menus haven't as many colours.
The Collection screen does not allow you to toggle 'effect' cards. See Cards for further changes.
As aforementioned, saving is non-functional. The quit function also can't return to the title screen; whether you choose to save or not, it'll just return to Base Camp menu.

The prisoner list adds a tally to the top, and lowers the mission name. The prototype only goes up to 25, claiming each page correlates to a mission, though the full list is still available in the ROM's data.
Prisoner 002 is misspelt as Betibase (ベティペイス) in the prototype's Japanese text; it was corrected to Petibase (ペティペイス) in the final.
Prisoner 023 is named Ector (エクトル) in the prototype across both languages, and Tanaka in the final.
Prisoner 037 is spelt Cardoc in the prototype's English text, and Cardok in the final.
Prisoner 049 Melias and prisoner 060 Meriot's English names were corrected in the final, to Merias and Meliot respectively.
Prisoner 090 also saw a rename in English, going from Balin in the proto to Suzuki in the final.

The mission select isn't fully cleaned up, with some blocky tiles and odd colours. The mission text border is discoloured.
The "mission start" screens have no text or voice yet, and pressing any button will instantly skip them.

As far as I'm aware, debug tools have yet to be implemented in the prototype -- I haven't found any RAM addresses pertaining to making collision boxes visible and the like, though certain seemingly unfinished objects will print data on-screen when they are spawned.

Gameplay

The ammo and bomb counters in the HUD are silver instead of gold.
The player's health bar is coloured orange in the prototype; it's green in the final, and orange is reserved for the Slugs' health. The Slugs have no visible health bar or cannon counter when they're piloted, and still display your character's health, though their health is still recorded at RAM address 03005640.

Speaking of health, Walter and Tyra's life metre is measured only in 5 hits, while the retail game measures it in 50, allowing for greater variables of damage they can receive. The player character has no mercy invincibility after their damage animation ends, and can immediately be hit again; the Metal Slug hasn't even that luxury, and will take damage during its damage animations.
Hitflash is not yet implemented for enemies, and they tend not to play explosions, instead disappearing when destroyed.

The pause menu does not dim the screen or stop the music, and the "return to Base Camp" option doesn't work. Pressing Select while paused will advance by 1 frame.
The card sprites are much smaller, seemingly edits of the arcade versions.
All items aside from weapons don't appear to be implemented: food items don't restore health, ammo and grenade boxes don't refill your stock, and Fire Bombs will not change your grenades. There's no sound for collecting them, and no pop-up for cards.

It's hard to tell if card effects are 'in effect' or not -- all weapons have the extra ammo you'd get by collecting them with the weapon Clip cards, and all hidden areas are open by default.

There are no area transitions; the moment it would normally fade out, it simply cuts to the next area and resets the music. You also have immediate control over the character, with no transitional animation of them running in.
The game is much more liable to ignoring stall points, not bothering to stop the screen or load the enemy. The final game isn't perfect about this either, but it's a smidge more reliable!

Dying will make the screen instantly fade to white; the final game will wait until your character has come to a stop or fallen off-screen before fading to black. The camera will still follow the player if they travel off-screen by any means, breaking the level boundaries.

There are no continues; dying once is a Game Over, and will reset to the title screen.

Enemy soldier behaviour is pretty much set in stone, though their reaction times in the prototype tend to be faster, particularly with bazooka soldiers blasting you the moment they appear on-screen. Knife soldiers will still attempt stab attacks while you are in the Metal Slug on occasion. Soldier's bazookas do not rotate after they are shot. Jumping against a shield will make you bounce, as if jumping on a tank.
Other enemies are a mixed bag; see Mission 2 and Mission 3 for details.

Vehicular enemies are a mixed bag, typically lacking collision damage when moving, but are largely operable. Girida-O tanks are prone to loading without their cannons, rendering them totally harmless! The Sarubia is often reluctant to stop and fire its cannon, instead preferring to keep advancing and pushing against the player; their body collision is also a little messed up, making it easy to jump over them. The R-Shobu flies up and away when destroyed, rather than plummeting to the ground in the final.

Missions

As aforementioned, the background tiles and palettes almost entirely got tweaked for the final release, so there are graphics changes abounds; see the respective mission pages for details, or my sprites page for raw map rips.

| Mission 1 | Mission 2 | Mission 3 |
| Mission 4 | Mission 5 | Dungeon |

I cannot find debug displays for hitboxes or level geometry in the prototype, so the placement of hotspots in my maps is not exactly accurate, and it's possible I may have overlooked some.

test stages

The first test stage is rows upon rows of crates containing cards, even clustered on the ground once it runs out of elevated platform... but without the corresponding message pop-up, we've no idea what any of these cards actually are. Getting close to the doors at the end will warp you to Mission 1.

The second test room is structurally the same, without the random squid, but with a dozen or so boxes full of cards in its back half. The doors will warp you to area 3 of Mission 1.

And the last one is unfinished even for a test room; all it has are two Girida-O tanks at the start and nothing else. This would be used to test every enemy under the sun in the final version.

The demo and How To Play screens are identical to the final -- that is, no different to the areas they're copied from.

The data for the internal level select lists every stage as simply "[no title]". The final game would name only a scant few of them, so it clearly wasn't high on their list of priorities.

Cards

The card collection screen is the most operable part of Base Camp, though as aforementioned, the cards themselves have no effect on gameplay; they cannot be toggled in this menu, and collecting them in gameplay changes nothing. Only the first seven cards have the toggle function visible in the proto, any others are missing it.

008 ~ 017
The name graphics on these cards were changed for the final, from X2 to Clip, and abbreviating the weapon name to only its initials.
Card 009: "Doubles rocket luncher ammo." Typo corrected in final.
Card 014: Its Japanese name is "E Cartridge" in the prototype, and "C Cartridge" in the final.
Card 017: "Doubles supply of grenedes." Typo corrected in final.

018 ~ 027
The name graphics were changed from "Pow Up" to "+", resulting in the names getting excessively crushed in the final.
Card 025: Another E and C swap in its Japanese name.

083
Proto: だい地震を発生きせることができるハカイ兵器
Final: だい地震を発生きせるハカイ兵器。

086 ~ 089
All the card graphics simply say "Soldier" in the prototype, while the final adds letters to distinguish them
The specifications for these cards are far lower in the prototype, spanning 20, 50, 100, and 200. The final ups it to 100, 200, 300, and 999.
Card 086: "Proof of courege. Must kill 20 foes." Typo and requirement corrected in final.

090 ~ 099
The same graphic change applies here. The prototype's English text also misspells hostages as "hosteges" on all of them.

100
"Special card for getting 99 cadrs." Not underlining this one as a special challenge to spot the prototype's typo!

object IDs

You'll want to get acquainted with the Cheat Engine tables to do some object swapping; I can't offer pointers for Mr_Beacon00 (信标先生)'s ones because I couldn't get them to run!
But the "no-pointer" table should work in Cheat Engine 7.4 and VisualBoyAdvance 1.8.0, provided you find an address yourself first and then recalculate new addresses properly. Load the "object swapping" savestate, make the changes you need, and hopefully after you unpause the first prisoner you see in 2-03 should be replaced...!

IDs are listed in decimal, and largely correlate with the final, though they do misalign by 5 around 180.

1 slug gunner??
2 black hound?
3 metal slug
4 slug gunner
5 black hound?
6 slug flyer
7 slug mariner (unused)
21 logo
60 plane launching missile
64 biker suicide cash
65 logo (glitched, scrolling)
66 ^
91 R-Shobu launching missile
127 mission 3 floor
128 mission 3 floating platform
131 3-02 wall?
132 3-02 wall?
133 3-02 wall?
134 mission 3 cave floor
139 5-13 wall
140 wall w/ hitbox
141 bg door
142 cell door overlay
143 discoloured wall
144 rock overlay
145 ^
146 ^
147 ^
148 ^
149 ^
150 ^
151 ^
152 ^
162 wooden platform
163 hut 1
164 hut 1 destroyed
165 hut 2
166 hut 3 destroyed
167 hut 3
168 hut 3 destroeyed
169 log bridge for mission 2 boss
170 ^
171 ^
172 ^
173 some kind of rope thing?
177 UNUSED HATCH
178 ^
179 ^
180 ^
181 a little orange dot...? disappears when shot
182 explosion, also disappears when shot
183 glowy dot, ditto
184 walter
185 tyra
186 japanese logo
187 PF cadet tied up
192 pf cadet in shackles UNUSED
198 soldier cannon + hut
199 wall cannon (203)
200 squid
201 sea mine
202 poison gas. animates constnatly, can't hurt you
203 ?
204 ?
205 FREEZE

ID: 1 ~ 7

Only the Metal Slug and Slug Flyer can be entered; the other vehicles can be spawned but not piloted, it seems.

ID: 91

The R-Shobu's missile launcher functions correctly in the prototype; in the final it uses the Metal Slug's graphics for its missile instead, for some reason...!

ID: 127

Still as intangible as it is in the final, but with a different palette!

ID: 128

Evidently what the Weird Ruins elevator originally looked like, with a far simpler design. It'll go upwards the instant you touch it, but it cannot carry the player.

ID: 131 ~ 133

The cave doors that block the hidden areas in 3-02, unseen because the proto hasn't coded them in yet. The floor portions are different from the file to account for the background tile changes. The third door is distinctly shorter, and it's not clear where it might have been used.

ID: 139

Similar to the walls in 5-13, but as a sprite layer. No collision or interaction.

ID: 140

A barred door. Can be shot but seems to be indestructible, yet can be walked through without a problem.

ID: 141

A sprite version of the cell doors seen in the background of 5-13. No collision or interaction.

ID: 142

A massive set of bars. Lines up well with object 140.

ID: 143

A discoloured chunk of wall, probably a foreground object for Mission 5.

ID: 144 ~ 153

What appear to be foreground objects that fit in with Mission 3's temple tileset and the underground tileset of Mission 5 / the dungeon. They come in various heights but are all functionally the same graphic. The game uses no foreground layers to my knowledge; the game's under enough pressure as it is without rendering superfluous detail like this...!

ID: 162

A two-tiered version of the wooden platforms in 1-01. Has no collision or variant state; being a sprite would suggest it's destructible, but the aforementioned versions are baked into the background.

ID: 163 ~ 168

A variety of destructible wooden huts! There's no sound or debris in place, but they can be wrecked by shooting them a little bit. They look at home in 2-02, as the end of that area has an edited version of the first hut. Your character can't otherwise interact with them, nor do they overlap the player, so unless they spawned enemies it's hard to tell what their purpose was.

ID: 169 ~ 172

Little chunks of log bridge. Only two of these appear in the mission 2 boss arena, leaving the others unused. They'd be ditched and replaced with planks baked into the background for the final, though they live on as background tiles in Mission 3.

ID: 173

Some kind of suspension cable? It's one of the few objects to have working animation, but has no collision or known hitbox. It's really long, and it's tricky to see the top of it without it despawning from being scrolled off-screen...!

ID: 198

A log-mounted cannon and a hut, seemingly an earlier version of the final's unused object 202.

That version lines up perfectly with this one, filling the gaps in the scaffolding sprite and mounting the third cannon on top of the hut. It has no code in place and is seemingly a mockup at this point.