This is the small black buffer that attaches to the rear portion of the RS/VS Gen3 balance valve. It absorbs a portion of the shock created when the striker clashes the rear surface of the valve. It's part of the reason you feel less recoil on these LP markers. You can use it as a replacement part on your Balance valve or use it on your existing valve of choice.

This is a replacement for Kingman part #ITP004 in your manual.