The article on Lee Navy rifles from the Winchester Collectors site mentions rifles marked USNM with a separate SN, not the Winchester-assigned SN.  The writer of the article wrote that he did not know what "USNM" meant, thought that it was US Naval Militia.  I do not think this is correct.  My thought is that USNM means United States Navy Magazine, a name used from at least Sp-AM War days up to the present.  Almost all such places had small arms in storage and often had repair capabilities.  Why some were called Naval Magazines and others Naval Ammunition Depots is a mystery to me.  I was stationed at NAD West Loch (HI) and Charleston (SC) and at Naval Magazine Guam and never observed any difference in operating procedures.