This is a standard rail button designed for the 1515 size 8020 aluminum extrusion rail. By being conformal and rounded they are designed to be more aerodynamic than the standard 1515 rail button by being rounded 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.