I have the same book, published by Honda.
Looking at both CB750s and 550s, it appears that about 1976 they abandoned the Kx designation which had been used on the CB750s.
Beginning in 1976, they seem to have embedded the actual year into the name, rather than K6 or K7 or whatever.
Thus we seem to have CB550K'76, CB550K'77 and CB550K'78 .Same thing in CB750K land.
I put some stock in this as it is Honda's own publication. Reference is page 231.