This is a Yankee Hill Machine Co Mount known as the 6" Picatinny Rail Extension, model YHM-9473.
These rails are perfect for mounting optics as well as any other picatinny style accessory. They add an extra mounting surface to any picatinny rail and also elevate the mounting surface 1/2." Made from aircraft quality aluminum and hardcoated to mil. specs.
Features:
Black finish
6 Picatinny rail extension;Mounts to any picatinny rail