This is the Ruger BX-15 magazine, a short version of the Ruger BX-25 released for use with the Ruger Charger pistol.
It makes a handy size for hunters wanting a bit more capacity than the standard 10 round magazine but without the 'full banana' of the BX-25 25 rounder.
Will fit any Ruger .22 LR 10/22, 77/22 or SR-22 rifle.
2025 update - Ruger have updated the design slightly, the magazines are now a snugger fit in 10/22s, closer to the BX-1 10 round magazine sizing, they have also added a rounded edge to the feed lips at the front. Please note the tensioning bar at the rear of the magazine is solid and can be modified if you prefer a looser fit.