This seal is used on both the MK1 and MK2 FX Panthera/Dynamic/King air rifles and provides a seal between the plenum end and the barrel housing.

If this o-ring is leaking, it is recommended to replace the two larger plenum seals: FX19485 Plenum tube seals.

MK1 vs MK2

Please note that the MK1 rifles have the plenum end with 2 locking screws.
These using 2 of these seals.

The MK2 rifles have the upgraded plenum end that do not have the screws.
These use only 1 of these seals.
These plenum ends fit the MK1 rifles and are available here: FX 20938 Valvetube End Plenum Dynamic