It's proprietary. This mount was for the extremely old NCstar line of "micro dot optics," catalogued as DDAB, DXGAB, and the rest. I don't know if they're still in production. Some people say that some Docter pattern sights fit this mount, and some DDAB sights will sometimes fit some Docter mounts, depending on manufacturing tolerances, but it is NOT a Docter pattern mount and you should not have the expectation that an optic made by anyone other than NCstar will fit it. That's probably why it's thirteen bucks.