Live cartridge stop is used to prevent 'live" i.e. cartridges with projectiles from being chambered in a 1919 with a blank firing adaptor installed. Live cartridge is first thing in when installing BFA and last out after removing. Crank off a real round with the BFA attached and you will regret it, and so will your 1919.I think that is the live cartidge stop for using G.I. flat nose blanks in a 1919. J