This is a Yankee Hill Machine Co Mount known as the 5 Slot Picatinny Angle Mount, model YHM-9691A.
These mounts are great when used with scopes" flashlights" lasers etc. Accessories are mounted at a 45 degree angle. When used with scopes the iron sights remain visible.
Features:
Black finish
2 1/8 in length;Fits Picatinny;5-slot angle mount