This is the forForearm tenon Winchester 1876 rifle. This part slides into a slot on the underside of the barrel and your two forForearm cap screws (wp56) screw into this and hold the forForearm cap in place. This tenon is drilled and tapped to accept the forForearm screws. If you have loose or new wood to fit and want to drill your own holes then use the tenon that has not been drilled or tapped.
