I've seen as many with the A1 birdcage as I ever did with the 3 prong. By the time I entered the USAF in 1982 all weapons on base had been converted to the A1 birdcage. Never saw the 3 prong until I came back home from overseas and started checking this out more closely at the gun shows. Most weapons by then had also been converted to A2-style handgrips. I cannot remember seeing an A1 stock that did not have a storage compartment and we still used the triangular hand guards on all M16(s) with the only exception being the carbines which used the large round double heat shield hand guards and none of the carbine barrels were under 14.5". Although I cannot exactly recall, it seems all uppers had the forward assist too.