The two Arkagis Revolution trial versions so far are revisions XZ and YX. If you paid attention, at Joypad the title screen said revision XY and at Gamescom it said XW. What's going on with these revision names?
It has in part to do with the Mega Drive ROM header format. The serial number field has a revision number going from 00 to 99. The problem is that this field is intended for released versions (00 being the first release), and these trial versions are pre-release (something Sega had never accounted for). I could have just started counting from there, but then the first proper release wouldn't be 00, which doesn't feel right.
Instead of doing that, I improvised a pre-release versioning scheme, using the letters X, Y, Z and W. This gives me up to 16 pre-release revisions (XX, XY, XZ, XW, YX, YY, YZ, YW, ZX, ZY, ZZ, ZW, WX, WY, WZ, WW). Feel free to copy it, I hope we never need to go beyond that! When the game finally launches for real, I'll switch back to the normal revision scheme (00, 01, 02, etc.).
You may also have noticed that I didn't mention which revision was XX. That was a privately distributed revision, and as such you're not going to have seen it in any form.