Well, I think it would be close. Having both a '74 and a '78 in the stable (fully assembled), I went and measured between the fork leg fender mounts on both. The '74 measured 5 1/8" between, the '78 measured 5 1/4" . I have the thing up on NH Craigslist as well, and after being quizzed by someone else I did a quick measure of it and got 5" between the brackets, and 2 3/8" between the mounting holes, bolt holes sized for an 8mm bolt.
So as close as I can see I would give it a definite "maybe". You should do some measuring on yours and compare.
I suspect quite strongly that it came off one of the "Specials".
PM me if interested.
Be well!