This is a standard rail button designed for the 1515 size 8020 aluminum extrusion rail. By being conformal they are designed to be more aerodynamic by conforming to the rocket airframe tubes in these standard sizes - 38mm (1.5 in.), 54mm (2.13 in.), 65mm (2.54 in.), 75mm (3 in.), 98mm (4 in.), 140mm (5.5 in.), 155mm (6 in.), and 195mm (7.5 in.).
You can download two versions of each airframe diameter - with and without a recess.