This extension increases the Plenum’s length by another 22cm, for a total of 57 cm. This provides you with more air for the pellet, which can result in higher velocities.
The Plenum extension is compatible with any of the Priest or Prophet’s versions. The total plenum volume when the plenum extension is installed on the Priest I, Priest II, or Prophet I, is 57cc, whereas for the Prophet II, results in 92cc. In fact, on the Prophet II there can be x2 plenum extensions installed simultaneously, resulting in a total volume of 114cc.