These images are sourced from Neo Geo Freak's April 2000 issue. Thanks to Division 六 for the images!
As far as I'm aware you can't enter the Slug Mariner from underwater, nor do you find one there.
It's a warp tube in Mission 2! It appears to be located where the CEO normally is; this would probably have led to the unused vertical area.
You find the Camel Slug in this area, not the Metal Slug.
These images are sourced from Neo Geo Freak's May 2000 issue. Thanks to Division 六 for the scans!
There's way more items than there usually are, including a Drop Shot, that isn't even found in this mission. It's only lady zombies at this spot in-game, but there are fat zombies and the medkits suggest scientist zombies here.
Another huge drop of items.
It seems the special prisoners were located in the ice cave at first; the CEO and Utan are seen here, but in-game are found on the main path long before the entrance to the cave. The right screenshot shows a Melty Honey - only zombies and icemen are found in the cave area.
Here's a strange one - it's Satiko Suzuki! Her graphic is used for a debug option in Metal Slug 2 and X as a indicator of where players will respawn... but 3 not only replaces it with a sprite of Marco, but you never respawn in the middle of the screen in the ice cave. What's it doing in an official publication?!
A lady zombie is here, but a scientist zombie takes its spot in-game; the ladies don't appear until a screen or two later.
These images are sourced from Neo Geo Freak's July 2000 issue. Thanks to Division 六 for the scans!
There's no hole visible in the ceiling, suggesting Tarma's just going to splat into it. Oof.
These images are sourced from Neo Geo Freak's August 2000 issue. Thanks to Division 六 for the images!
The bird item is replaced with a cat in the final.
These images are sourced from the American arcade flyer.
It's probably just odd compression or something, but the area seems very blue in the screenshot, while the final is green. Marco is missing the empty energy gauge in his HUD, there isn't a Heavy Machine Gun in that area in the final, and the giant eel always seems to have it's mouth closed when it enters the cave in the final.
Probably another compression error, as the colours seem to have blue and red tints, and the clouds seem less detailed. The missing prisoner and HUD with lack of energy gauge seem to be the only other differences.
There aren't any suicide bombers or mines when you're fighting the giant submarines, and of course, missing the energy gauge.
The crabs look more red than orange, but as I've said a million times, it's probably just a compression error. The HUDs are, once again, missing their energy gauges, and it looks like the battle with the two crabs took place elsewhere in the area, what with the background being different, along with there not be boxes there.
Once again, the colours are off, and the energy gauges are missing, but what's interesting is that there are four soldiers on the enemies' boat. In the final, there's only ever two people on the boat, with the second coming out after the first one chickens out and drowns himself. It's also worth noting how close together those four grenades are, despite there being only three visible soldiers who could've thrown them, and they would've had to get out and come to the other side at roughly the same time to get them that close.