A manual issued at the 1963 National Matches indicated that the “NM” on the barrel in the vicinity of the gas cylinder was “inscribed” not stamped and was added only after the complete rifle was assembled and passed the targeting and final inspection.
After the M1 was superceded by the M14icon, I knew a retired armorer who evidently took a number of M1 NM replacement barrels with him. I bought 4 or 5 for $20 each. (Read it and weep). All of them were marked NM on the chamber with all of the other barrel data. I believe that NM barrels were standard M1 barrels that passed the straightness test and the air gauge test. My 1963 NM carries the “NM” marking in both places (Chamber and front in the vicinity of gas cylinder). I have a match rifle (not NM) that was built by this armorer for himself and it has the chamber NM mark only. He was transferred overseas so sold me the rifle before he could use it.
I am sure that someone else on this board will be able to provide better info. My observations are based on my observations as an outsider not as an armorer or historian.