First question is, was it purchased as a frame/fork from a bike shop or from Masi? If either one is true, the first installed set of rims might have nothing to do with what you got if you bought a fully factory-built GC from the Masi factory or from a dealer.

Bottom line, any rim made in 1975 or perhaps up to 5 years earlier (found in the "supply cabinet" of the wheel builder) could plausibly have been the first set of rims installed on the bike. I don't think the word "original" means much if you don't know how the bike was originally supplied.

Just to noodle out a plausible response, should they be light rims (legero, I assume) or heavier training rims (the word "strada" suggests that it's for the regular roads)? I doubt grooved sidewalls were very common.