You need to contact an expert on historic collectables. You will need to have photographs of at least a couple of the existing plates (front and back) to send. There was a Los Angeles company, Vernon Kilns, which producted collectable pottery and did have California Mission plates. You might check the back of the plates to see what information is there. In order to fill out the collection (presumeably there are at least 21 plates in the set) you will have to work with an antique dealer or go onto ebay. It doesn't look to be terribly expensive.
Taxonomy upgrade extras: