C1/C1A1 TMH assemblies were available in quantity many years ago from places like Sarco and Numrich. We built a couple of the rifles including the one I still have using early Entreprise receivers and Australian barrels since Canadian barrels other than the C2 heavies were never available in the lower 48 in quantity. Like Peter, I'd guess your TMH was numbered and proofed to match by the commercial builder although I suppose some used ones may have been available with those markings. All of the ones we bought at the time for our builds were nos and unnumbered. I never endeavored to have C1A1 markings or proofs engraved on mine. It is what it is and shoots great.