1-1/2" keyseat cutter, 5/16" face width, 16 flutes.
A keyseat (Woodruff) cutter for milling keyways, slots and undercuts. It cuts 1-1/2" diameter with a 5/16" face width on a 1/2" shank, 2-5/16" overall.
The staggered teeth cut on the periphery, so the tool feeds sideways into an existing opening rather than plunging.