This is the carrier Spring screw Winchester lever action model 1886 rifle. This screw hold on the carrier spring on the underside of the upper tang. (Until serial numbers 19,500-23,500 the carrier spring screw was visible on the upper tang because the hole was bored through, behind the front tang sight mounting hole. After that the hole was not drilled through the upper tang) NOTE: sometimes the carrier spring is mistakenly called the carrier stop and thus carrier spring screw is labeled as a carrier stop screw.