5/8" keyseat cutter, 1/8" face width, 10 flutes.
A keyseat (Woodruff) cutter for milling keyways, slots and undercuts. It cuts 5/8" diameter with a 1/8" face width on a 1/2" shank, 2-1/8" overall.
The staggered teeth cut on the periphery, so the tool feeds sideways into an existing opening rather than plunging.