This O-ring is a factory replacement part used in several Bertolini piston semi-hydraulic diaphragm pump models. It is commonly included in standard valve assembly kits and pump seal overhaul kits:
Bertolini PA 530 (and variants like the APL0540KGH 54L/min pump assembly)
Bertolini PA 730.1
Bertolini PA 830
Bertolini PPS 100
