 On cheaper mounts, the flat surface may be simply the material of the mount.In more expensive mounts, the flat surface (and perhaps the hole and v-groove too) may be made out of a much harder material (often sapphire), set into the frame.The reason for this strange mechanism, is that the first ball (ideally) makes contact with the fixed frame at exactly three points, the second ball at two, and the third ball at just one.