This is a standard rail button designed for the 1010 size 8020 aluminum extrusion rail that is rounded and is conformal. By being conformal and rounded they are designed to be even more aerodynamic than the standard conformal button and 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.