This O-ring goes around the Regulator Piston (parts #684 and #822). These are included in all parts kits. Typically this oring is replaced during a complete service of the gun or when  the marker is having velocity problems If this oring is failing gas may leak from the back of the velocity adjuster. That leak may also signify a piston failure if you replace it and the gun still leaks through the adjuster. Sold in packs of 4.