For people who have lost or damaged the small CNC-Aluminium Wedge piece that fits into the hole drilled in the lower receiver when installing a Zenith kit.
It's purpose is to fill the gap between the lower receiver and the Receiver cap so the whole assembly is tight when the body bolt is tightened, helps transfer impact shock from the lower to the upper receiver. It also reduces wear on the plastic lower receiver from repeated opening and closing of the gun.