California started registering autos in 1905. Owners would get a small brass tag with the registration number on it. This would be attached to the car and the owner would make a larger plate with the number on it to hang on the back of the car. These brass tags were issued from 1905 up to 1915 and maybe beyond. I'm starting a collection of these and would appreciate hearing if you find one in your visits to swap meets. Here's a picture of one.