This is a Yankee Hill Machine Co Mount known as the Mini Riser, model YHM-226.
These risers meet mil. spec 1913 and use a larger screw for more clamping power. They fit any picatinny rail. Great for elevating scopes" sights" flashlights" etc. These risers elevate your accessories 1/2"
Features:
Aluminum construction
1-1/4 long