1/2" x 7/8" x MT3 DUAL BAYONET DRIVE SPOT FACER PILOT ARBOR TOOL HOLDER PQ 6190
Spot Facer Holder. MT3 - #3 Morse Taper Upper Shank. For use holding spot facers or countersinks with 1/2" lower drive which appears to be a dual bayonet lock or balanced drive. (.395" at Major Flats).