if you check the parts books you will find seven 750 frames with differing numbers, not all of them with major differences, the later ones its just bracketing etc, but the frame tubes near the steering head of very early 750's is quite different from SF!'s onwards,as is the tbing just rearward of thegearbox, plenty have found that carbs (or more like trumpets and filters) wont fit because of this, the changes seem to be to have been largely to lower the production costs, quicker to make and weld etc, as well as updates as time went by, there is proabably a drawing of the frames either on the wall or in a cupboard at Ricardo Oro'
CLEMs