It's certainly a pre-release version, which is very rare.
My theory is that they may have found a critical bug on 910204 after starting mass production with this revision. Therefore they stopped production, fixed the bug then re-started production 2 days later with fixed version (910206). That may be why 910204 are very rare, unknown and undumped because only a few pieces were produced and maybe not sold in 1991 but kept in a warehouse then sold years later by people who decided to get rid of them.
Anyway, whatever the story is, I'm a proud owner of a rare version of my all-time favorite game.