.312 x 7/16 x MT2 NATIONAL DRIVE BACK INVERTED SPOT FACER PILOT ARBOR TOOLHOLDER
Morse Taper 2 / MT2. Believe the part number may be # 51303 ?. For use holding in spot facers cutters or countersinks with 2 spline. 312, National drive # 312. Marked ".312-H [National Logo] 7/16 PAT#2120623".