[Edward Tinker]

what are the proof and acceptance markings on the right?

Although the sn is not on the frame, the regulations just state it has to have a serial number stamped conspicuously...

(c) In the event the firearm does not bear a serial number, the applicant shall
obtain a serial number from the Regional director (compliance) and shall stamp (impress) or otherwise
conspicuously place such serial number on the firearm in a manner not susceptible of being readily
obliterated, altered or removed.

Also, prior to this ruling and regulation, serial numbers were not required:

The Gun Control Act of 1968 (GCA), at Title 18, United States Code (U.S.C.), section 923(i), and the National Firearms Act (NFA), at Title 26, U.S.C., section 5842(a), require all licensed importers and manufacturers to identify each firearm imported or manufactured by means of a serial number engraved or cast on the frame or receiver of the
weapon, in such manner as the Attorney General shall by regulations prescribe. Federal regulations at 27 CFR 478.92(a) and 479.102(a) prescribe the requirements for

serialization and other marks of identification that must be placed on firearms.

Ed