A simple washer of sufficient size can cover the larger recessed nut hole and allow a traditional long center bolt with common nut to secure the brake. However this can also loosen more easily allowing the caliper to move about. I have seen said washer custom made to have a neck which fit within the larger hole to help locate the center bolt. Looked to have been made on a lathe by the rider. Of course there a number of other aspects must be correct for the caliper to fit well and do it's job. Andy.