A question came up on how the peep sight swing arm is retained in the elevation arm. Maybe a little more detail will help. Lyman used a small screw (verified by Fred, thanks). When I first made one of these peep sights, back in the spring for my MS M1910, I first created a 3D model to test my design thoughts. So I have attached a section view from that 3D model which clearly shows how the swing arm is always captured even while swinging the arm around 360 degrees.