Green CO2 bulb seal for the Bruni 1911 US Combat CO2 Air Pistol. Only fits the US combat version with the external screw like this:

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.