White CO2 bulb seal for the 1911 blowback air pistol by Crosman.

This is the seal against which the CO2 capsule fits and the most common to be replaced.
Generally needs to be replaced if a CO2 capsule has been left in the airgun for a long time, which causes leaks when the CO2 is replaced.

The most common cause of leaks on these pistols.

Now only available as an assembly with the retainer and o-ring