To Quote from the handbook issued at the 1963 National Matches:
“20. Identification mark shall consist of the letters “NM’ approximately 1/8 inch high "inscribed" on the barrel between the front hand guard and front sight.”
Mine is on the left side but I believe that some were inscribed on the top of the barrel. The gas cylinder "NM" appears to be stamped. My NM is a fully documented NM that I bought through the DCM in 1962
For what it's worth