This rack is clearly stamped Honda CB500K1 1972. The CB500K0 K1 is a carbon copy of the later model CB550 Four through 1976 Model Code 374. I believe this rack will fit all of those models, although you may have to remove the stock frame rail in the rear.
I also believe it will fit a 1975 and 1976 CB550 Four with the frame rails removed.
If you have ANY questions about fitment for your particular model, you can email me and we can take measurements before you order.