. Your 74 should not have a third pin. If it even has the hole, it is legally a machine gun, and needs destroyed. Unless of course you are licensed to manufacture NFA firearms. Even a hole that is plugged, is still a hole. Once a machine gun, always a machine gun until destroyed. You dont even need any FA parts. If it has the hole, thats all it takes.
A 74 should be no different than a 47 using that plate. If you are looking at the coiled spring with a long tail, that is the auto sear spring, and the tail functions as the wire to lock in the pins. That is not the one that you use in your 74, unless it is full auto.