Old Part Numbers: SSF-3158-1This is the screw that holds down the piston cover, on the top of the piston.

Air compressor check valve. 3/8" Comp x 1/2" MPT with 1/8" FPT side port.  The check valve is a one-way valve that prevents air from back-feeding from the air tank. If your unit has a hard time restarting when trying to cycle or your pressure switch...

Head O-ring Gasket. This is the o-ring on top of the valve plate, directly underneath the head. Order this if ordering valve plate or a new head.