Green CO2 bulb seal for many Borner air pistols. If it has a green main CO2 seal this will be the one you require.

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.

Fits: 
Borner Sport 702, 703, 708
Borner PM-X
Borner C41
Borner PM49
And many other models.