While like you, I doubt its a prototype (especially since it has FCC approval), its certainly a neat piece of history - I've never heard of there being post-prototype-but-pre-production Macs before. I actually went to an Apple roadshow in 1999, only a small one, that was done by a local AASP, they had the slot load iMac and the G4 there, but they were just ordinary production model systems taken out of the showroom.

I take it there's nothing inside that identifies it as a prototype?