"Will a 77 F's dual brakes fit on a 73's forks?" In a word, No. The '77 had comstar wheels, and the calipers were mounted on the trailing side of the fork (on the rear side of the fork). Since they had the comstar wheels, the rotors are different with the flange to disc offset, and the bolt hole pattern is different too.
So, to answer your question directly, yes the fork tubes are differnt.
