I was looking at the rear fenders on CB750 K6 and my K2 CB500, it looks like they will interchange. There are differences, like tail light wire holes and clips that hold wires, other than that it looks like they will bolt up.
Opinions......... I would like to get a really nice rear fender for my 500, Ive seen new 750 fenders. The one one now has rust pitting. Maybe get it re-chromed, not sure of the cost to have that done.