KP is right, a 500/4 stand will fit onto a CB750 (they use the same pivot pin), but whether the 750 would sit right on it, no idea. However, owning both 500 and 750,s, I can say with certainty that the 500 stand is NOT just a 750 one without the brace - it differs significantly in detail when you get them side by side.
SAD TALE - Friend of mine, restoring a K2, orders a NOS mainstand from Western Hills Honda. When it arrives, he is p******, cos they have sent him a defective one - no reinforcing strut. Because he can't be bothered to ship it back to the US, he sets to and welds in a piece of bar. Fantastic job he did too.
Wish he had rung me first, I would of swapped with him a NOS late type and saved him the welding job.

Proves though that early type no strut mainstands existed as spares, hence some later bikes could easily of had them fitted over the years.
Chris R.