On Motogrid's fiche the part number is missing for the CB500 and the CB500K1, but it's there for the CB500K2. All the drawings appear to be the same. It looks like it's just a case of missing data. Weird.
Anyway, the part number is 45140-323-670 and it is common to the CB500's and CB550's. It appears to still be available new, but I'm sure some members have one or two laying around (I have one in a parts pile somewhere).
~john