Thanks for the info, Keith & Tom.  A little deeper to dig into the engine than I was hoping to do, but that's something definitive to look for to see if the engine has had the upgrade.

And thanks especially for the parts list, Keith.  No, I guess I really wasn't planning on using that retirement account for retirement... 

I rescued one of my sandcasts from a breaker as basically just the motor and frame.  The frame is definitely set up as a CR.  It could be somebody's pretty well done mod of a standard CB750, but some of the little tabs and nibs are different from both my earlier CB750 sandcast (frame# about 600 units earlier) and my 10/69 diecast (frame# about 3500 units later).  Those aren't the sort of things I'd imagine somebody changing in building their own CR, and has me wondering if this could be an original factory bike.  Doubtful, but some aren't accounted for.

The frame number starts with the usual CB750-1005xxx.  Did the factory CRs use the same numbering scheme for the frame?  Or no numbers?

The engine number also looks normal, CB750E-1005xxx, where xxx is about -100 units different from the frame.  Is there anything in the engine that would easily identify it as being CR-spec that I could verify without digging *too* far into it?

My plan is to restore it as a CR since the frame is already set up that way, but it'd be extra-special if I could determine how much of it is the real deal.  

Mr. Bill

From the stories I've heard on other vintage car/bike forums on CA plate problems like this, your best bet is to look for an old registration card showing that tag being previously registered to that bike. Hopefully you've got one squirrelled away somewhere with the paperwork for the bike. They'll want proof positive that the tag and plate were originally mated together. No tickie, no washie.