Hi Kieth, i think there are 4 different frames for the CB750,
1st the K series 69 to 75/76, then the F1 76/77, then the F2 77/78, and finally the K7 77/78,
i could be wrong with my years etc but others with more knowledge will hopefully put me right

,
i think the biggest difference would be the earlier bikes were drum brake on the rear & the f1 & f2 were disc & the k7 went back to drum also the exhaust mounts would be different the earlier bikes had 4 into 4's & the f1 & f2 had 4 into 1's the k7 4 into 4's,
hope i have helped you a little

cheers Mick.