Yeah, apperently theres 4 different revisions. Found this on DigitalPress.
* Software revision 1.00 (1989): Bosses clearly resembling The Terminator (The Hulk when nearly defeated), Godzilla, Spider-Man, and Batman are present. Both Spider-Man and Batman are actually fake representations of themselves conducted by someone able to morph into his shape when enough damage is inflicted.
* Software revision 1.01 (1989): Batman is replaced by a bat-like mutant, but The Terminator/Hulk, Godzilla, and Spider-Man remain unmodified, the latter still morphing into the bat-like creature when defeated. Enemy soldiers with flamethrowers are also changed to bald men with headbands away from their original likeness to Rambo.
* Software revision 1.02 (1990): The fake Spider-Man is now redesigned into the licensed character from Marvel Comics, acknowledged by a copyright notice (Sega was also developing The Amazing Spider-Man vs. The Kingpin at the time). Since he is now the real Spider-Man, he doesn't morph into a bat-like shape and actually leaves the battle when enough damage is inflicted, with a separate bat-like mutant entering to replace him. The Terminator/Hulk and Godzilla still remain intact.
* Software revision 1.03 (1990): Godzilla is replaced by a skeletal dinosaur. Everything else remains intact from the previous revision, including the licensed use of Spider-Man. The Terminator/Hulk was still the same and he has never been affected by copyright issues because of resemblances with the real characters. This version was used in the Genesis 6-Pak cartridge.The Hulk may have not changed because of Sega having the Spider-man license.
Mine, unfortunetly is one of the last two revisions, havent checked to see yet. But I remember trading a really beat up copy to a friend, so maybe that could be an original, Ill have to check next time im there. I do know there is one way to tell the difference between older copies and Sega Classic copies tho. With my experience, most the Sega Classics releases Ive come across seem to have shiny laminated labels indicating a later production as where the older ones (like most older release genesis labels) are unlaminated. Might not be able to tell over a ebay photo but at least if you ever see one in person with a nice shiny laminated label, chances are its a Sega Classics revision.