Based on the old P.I. Spence design, this sight features a 5" long tang that mounts on the top barrel flat just ahead of the breech plug. The aperture unit rests above the breech plug tang. Threaded 7/32x40 for most peep sight discs such as the Lyman disc (with 5/8" diameter) which is included. Screw adjustment for windage and elevation. Minimum effective height is .310". Barrels equipped with a hook breech cannot be removed with this sight in place. Requires mounting screw holes drilled and tapped. No. 50100