Saco Lowell did make NM USGI barrels in both standard and medium weight. I own three SAK marked NM std weight barrels and two NM marked medium weight bbls. The medium contour bbls were made in the
early '80's and have SAK's cage number, bbl drawing number, and date of manufacture, as well as the M and P chamber area markings found on other USGI bbls. A couple of these bbls have sold on ebay in the past few months for about $400-$550.
One of my medium weight bbl's is on a preban SA Inc. receiver that was shipped in Jan of '86 and has the inscription "1 OF 100" stamped on the right side of the receiver heel. I haven't figured that one out yet.