This clear urethane washer sits at the base of your power tube.
When the bolt returns, it hits this to decelerate. This bumper is a little softer than the standard blue bumper for the higher firing rate of the Retro Valve. Should be replaced about once a year. Now sold in packs of 2.  Although this bumper wears faster than the blue it should be used in all level 10 applications and even in level 7 application where the gun is set to be very reactive.  The softer bumper creates less bounce back/chatter for the bolt and can save wear on your sear and bolt.