BSA Piston O Ring.
This spare fits the following models:
Airsporter Mk 5
Airsporter Mk 6
Airsporter Mk 7
Airsporter Centenary
Airsporter Stutzen
Airsporter S Carbine
Airsporter S Mk 3
Buccaneer
Mercury Mk 1
Mercury Mk 2
Mercury Mk 3
Mercury Mk 4
Mercury S
Mercury Challenger
Mercury 365 Magnum
Scorpion