Nothing special about the NM slides other than they had reduced internal dimensions to allow fitting to the frame and bushing. The GI slides had loose tolerances to fit on any GI frame they were installed on.
No confusion. It is a 1960's vintage replacement slide. The military started experimenting with hardened slides in late WWII, and the replacement slides beginning in the 1950's were hardened. You will never be able to tell the difference.