Leave itto the govt to have a dumb [censored] rule like that.
However, I think you can put the vintage plate on the front of the car.
Ooh wait a minute ... got a better idea.
Go back to registries (AMA, they can think while standing) and tell them you were the original owner of the plates. They have no way to confirm it as the records date back to the early eighties. It just a standard question they ask and has more to do with current plates
That's a good idea! The only issue is in 1969 I wouldn't have been old enough to have registered them (barely). Geeez, first time I've felt "young" in a long time!