Apparently the first Solingen fighters could be late 70's. There may have been a couple of trial runs according to GTR. .

I have a fellow that says he bought one in 1976 and remembers it because it was the bicentennial. Doing some additional checking, and trying to get some photos of a one with the three thick two thin spacer arrangement. 1976 would seem to be a bit early for that model IMO, as we were totally out of Viet Nam in 1975, and Solingen 14's and 15's were still available for a couple of years after that date. So not 100% on it right now.