Green CO2 bulb seal for the Smith & Wesson M&P 9 4.5mm BB air pistol by Umarex.

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.