This is a new old stock, five-fluted countersink bit for your brace. This one has the 3/4" width diameter at the widest point.  Able to cut a countersink to a 3/4"capacity. It is marked "STANLEY MADE IN U.S.A. NO. 139". These are soon becoming rare in new condition at least. I only have a few of these left. Price is per bit.