An update! The PUMA V2 has a ROM version of "A2 H" (thanks to the Discord user that sent me this info!)
This one's a bit weird - the other known V2 versions are A.3, A.4 and A.5 (though there's at least another two versions on top of those). The PUMA version doesn't have a period between the A and the number, meaning this was probably the start of the convention that the V3 used, where the retail version number follows the letter A. What I'm not sure of the meaning of is the H - I don't have a clue why they chose this particular letter.
Hopefully we'll be hearing about what ROM version the GLAY Expo Plus used soon - if it's anything like the PUMA version I'm sure it'll contain a random letter too.