This is pretty cool, never seen that before. I wonder if weapons and munitions factories had these for security reasons ie. fifth columnists/enemy saboteurs, criminals etc. I know the media in this country like to play down this threat, ie Japanese and other nationalities residing in Canada and the possibility of enemy saboteurs. When one digs into the personal files of many so called harmless first and second generation Japanese, Germans etc. you would be surprised how many were involved directly with supporting what was going on back home. This also includes Communists during and after the war. There was also the minor threat of POW's escaping and somehow managing to conduct some kind of sabotage. Anyway I would bet that this is what this mark is for. Just to ID the weapon as not being part of the stock but a weapon relegated for factory defense.