Dave the picture is dated March 1942 and at that time the projected production of the trigger housing was to have a built in castle nut wrench. This was dropped shortly after that was published as the military realized if the primers were non corrosive then there would be little or no build up in the gas cylinder with fouling. And the clumsy GI would not be loosing a castle nut or piston if they were taking it apart to clean it. Clearer heads prevailed.