I have an M&P full size pistol that I was installing an Apex sear. When I pulled the sear housing there was a little block about half the size of the sear on the left side of the housing. It fell out as well as the spring. What does this thing do? I noticed in several youtube videos and images online that do not have this piece in it. My gun has the thumb safety that I removed and I suspect this is part of it correct?