Green CO2 bulb seal for the Umarex Smith and Wesson Model 629 air pistol.
Also fits S&W 629 .44 Magnum Competitor

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.