MODEL "123" BACK PRESSURE RELIEF REGULATOR,TFE GASKETS (OPT-45) 1" SIZE; BRONZE BODY & SPRING CHAMBER; B4 TRIM: BRASS WITH TFE SEAT,FLUOROCARBON ELASTOMER (FKM) DIAPHRAGM; NPT SCREWED; 165-350 PSIG RANGE SPRING; OPT-56: SPECIAL CLEAN PER SPEC #S-1542;