The "C" prefix serial numbers were in fact made specifically for California back just prior to the Roberti-Roos ban. They were made in large quantities specifically for sale into the state prior to the passing of the RR ban because at that time, we, and no other manufacturer (except for Colt's) was sure what the California DoJ was going to do with regards to interpretation of the bill. There was still a possibility (prior to the passing of the bill) that ALL AR's would be banned including of course, all Olympic brand.
SO... we made them as fast as we could, and were even at the time able to ship them into ca just as raw forgings with a serial number and have that count as legal "gun". Completely unmachined, but with a serial number. We sent them in, they were sold by the dealers, then the customers sent them back to us for machining.
Other than that, there is nothing special about this receiver. It is the same as all the others we have done. The only differences are that it has a special serial number to help us at Olympic identify the receivers.
They are of no special collectors value, we made literally thousands, and there is nothing that would make them any different than any other AR receiver in California with regards to SB23.
Tom S.
Oly Arms