Its an Apel mount. A 13mm one in this case. They did them for HWs (11 & 13mm), FWB Sport/Omega and BSA Mercury/Airsporter with a relieved scope rail for better eye relief. They are adjustable for windage and ring height and come up for sale s/h now and then. I usually buy them if I see them in time. http://smg.photobucket.com/user/Airs...ml?sort=6&o=68
Cheers Dave.