Fixed 1 Piece Medium Picatinny/Weaver (Top) 11mm (Base) Riser

This adapter works best with 2 pc. Weaver mounts. The Weaver grooves are evenly spaced which allows for a placing 2 pc. Weaver mounts at virtually any distance apart.
It will also work with some Weaver 1 pc. mounts but only if the mount allows for one of the cross pins to be moved slightly forward or backwards. Some Weaver 1 pc mounts allow for adjusting and some don't. Make sure your mount allows for that prior to ordering this adapter.
This adapter also will work with 3/8 inch rails or dovetails.