I would imagine that these are simply the different builds/production groups for the Mouse. In general, functionality of the Mighty Mouse hasn't chagned since its inception in 2005. Perhaps minor component changes, but in general you should be good with all versions. The most recent inception in 2007 didn't minor cosmetic (changed the color of the side buttons to a grey, so if you see one like that it would be the most up-to date) adjustments but again, does not effect functionality. I hope this helps.
-Josh