This is a leveling foot designed to fit a pair of 2020 extruded aluminum rails, as shown in the photos of the back corner of a 3018 CNC mill. The lugs on the back of the part engage the rail slots.

The hexagonal pocket in the bottom takes an M4 nylock nut.

The mounting holes will work with either M3 or M4 screws.

Best printed with the hexagonal pocket down. It may be necessary to file the bottom edges of the two lugs if there's some sagging but it should be minimal.