Plates are governed by each State, each State being independent of the other. Your only concern is California, as you said it came with a California plate.
I live in, and grew up here in sunny Southern California. I am by no means an expert. In California, the plate belongs to the car and stays with it as it gets sold to the next person. You should contact the DMV (Department of Motor Vehicles) of California. Using the VIN and the Plate numbers, you should be able to trace it's history. I don't know of any private agency that does this, but....
The link to the California DMV is:
California Department of Motor Vehicles
My best guess is that a US Military person took it with them while stationed in England and sold it.