You could if you've still got it scan the 'Customer's Reciept' portion of the money order. That would at least prove you purchased it. They have dates on em too so it'd be definite proof.
From my experience with the roms, the JPN version of Progear is the better, and apparently cheaper way to go. The US version lacks the speech at the end of the levels and I think the default difficulty was changed, not sure. I read the number of US boards produced was 30, not 13, but that was just on a messageboard
I'll definitely be looking to get Progear at some point in the future since it's one of my favourites in mame at this point. First I need to find an RGB supergun with psx inputs for my arcade stick...