Prior to the statewide bike registration program launched in the mid-70s by the DMV, individual cities and counties who offered local registration were somehow issued what I believe were called "bugs", that is a metal stamp which when pounded into the bottom bracket applied that jurisdiction's "symbol". The symbols varied from various common shapes to a letter or two that would be an abbreviation of the city or county. I would see one now and then on a student's vintage bike when looking for the serial number.
When I was in charge of the bicycle registration program at UC Davis I had an old (probably 1960s) publication that was probably titled something like "California bicycle registration guide". It was a thin 8.5" x 11" paperback that I recall was written by a police officer. It included a list of all those marks and the locations to which they were assigned. I sent an email to my successor in the job and have asked him to see if it's still in the bike office. If so, I'll get it and report back to you --probably no earlier than next week.
Incidentally, prior to the statewide California licensing system, many cities issued an actual small metal license plate with an embossed license number, city name and a date. I framed a collection of them from across the state that were given to me by a retired UC Davis LEO who had been in charge of the registration before me. I believe he had removed them from abandoned bikes collected from around the campus.
David Takemoto-Weerts
Davis, CA