I just picked up a new to me Model 64 in excellent condition. It has the marking "MC72" on the left side of the frame under the cylinder. It is in the same style as the markings on the California guns that came on the market last year (CDCxxxx and CYA). Anyone know what the MC72 means? Are these marking done by Smith and Wesson at the factory for contract guns or by the agency that buys them?