This is the dust cover (mortise cover) spring Winchester 1873. It is held in place by a screw (wp607A) in a rectangular cut out on the front of the dust cover rail. This little spring sticks up out of the rail and puts tension on the underside of the dust cover to help keep it closed (after you push the dust cover back with your finger) and to keep it from going back and forth when tipping the rifle. Your dust cover has to be removed in order to see/replace this little guy.
