Green CO2 bulb seal for Umarex Colt 1911 model air pistols fitted with this green seal type – NOT suitable for those with the white.

MUST be this green seal:

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.