In the US, there are K0, K1, and K2 models of CB500; 71,72,and 73 year models
Europe got a K3 CB500 that was simply 50 less cc than a US K3 CB550 model for 77-78.
The difference in the engine units between these models K0,K1,K2 vs., K3 is vast
We will probably need to see pictures, or see frame/engine numbers to sort out if you have 77 import K3, or an earlier model government stamped with the year of importation without regard to actual manufacture date.
Cheers,