Original SA 1943 annealed receivers

[RCS]

valve screw assembly B147851 isued with the M7 launcher in 1943, replaced by the poppet assembly in Jan 1944.

The early valve screw vented the gas when launching grenades or ground signals and also prevented the M1 from semi auto operation with ball cartridges (as the gas system was vented by the M7) until the M7 was
removed.

My question, how is the bolt (when launching a grenade or ground signal) making contact with the receiver heel when the gas system is being vented
by the valve screw and M7 ?