Evidently the numbering sequence that California used in the '30's and later means nothing... I have a complete set of CA plates from 1923-1940... which came out of a barn in the foothills above Colfax CA which my brother in-law purchased in 1973. All of the plates I have, staring with '29, have a "D" or "E". I had assumed that the "D" & "E" were specific for that area... WRONG
I have a '39 conv that has a '39 CA YOM, #12 M 000.. I have a '40 Buick which I wanted to put a '40 YOM on.... I wanted the YOM to be #12 M 000...
While looking for such a plate I was told that The CA DMV sent plates out to their various offices in groups of consecutive numbers with no regard to a letter or number being specific to any given area, however, it can be assumed that the letters are area specific, the trick would be in backtracking to find out where the plate was originally registered to a vehicle.
The numbering system used from 1929-39 all start with a number. then a letter, then 3 to 4 numbers.
I found a pair of 1940 CA plates a couple of months ago with the 12 M 000 numbering to use on my '40 Buick .. The three digits are different from my '39 plate by a 100+ numbers.