This is a diopter set to place on your air rifle. This diopter fits an 11 mm Dovetail Rail.
SANLIDA RECURVE CROSSBOW SETS CHACE WIND BLACK 150 LBS
GAMO Match Grade Diopter
Feinwerkbau Oberndorf Diopter (11mm Rail)
Krico DRM - Diopter for Rifle