I have a "51 Stude Starlight Coupe with Calif "Black Plates" These are
the 1963 issue (with the black background and yellow letters) they
replaced the earlier plates issued that had the yellow background with
black letters.
As far as I remember prior to 1963 the plates were the yellow with
black letters.
If your looking for Calif 1951 plates you could watch ebay for them.
Be advised that Calif requires two plates so you'll need a matching
pair.

Are you in Ca?  If you are in Ca I can answer your question:  Can you
prove the old 63 plates you have on the car belong there with a old
registration card?  If so you can continue to use those same plates;
free of charge, with a statement of facts that you want to continue
those plates(YOU MUST HAVE PROOF, other then the plates being on the
car).  If you are looking for YOM Plates: Year of Manufacture plates -
you must find a clear set that match the year of your vehicle. They
are oversized black with yellow letters.  One of us here just happens
to be a Manager at the CA DMV
Rhonda
(ps DO NOT SELL THOSE 63 plates, they will be very valuable next year
when the law changes)