1-1/2" keyseat cutter, 1/4" face width, 16 flutes.
A keyseat (Woodruff) cutter for milling keyways, slots and undercuts. It cuts 1-1/2" diameter with a 1/4" face width on a 1/2" shank, 2-1/4" overall.
The staggered teeth cut on the periphery, so the tool feeds sideways into an existing opening rather than plunging.