I went out and scoured my California registered 78 CB550K. And, I find no EPA sticker on it. I don't believe there ever was one, and I find no place where it would have been.
I have a 79 CX500 that does have a sticker/plaque for EPA specs/ and settings pinned to the frame.
Almost certainly, DMV is just dicking you around. Or, there have been some(more) recent law changes the Dems have put in place to make life more miserable for us and show us just who is boss. Probably have to find a referee center and pay to slap some sense into DMV.
Make sure the bike is being registered as a 78 and not a 79.
I just cannot get out of Kalifornia fast enough.