Model "123" Back Pressure Relief Regulator,Graphite/Nbr Gaskets; 1/2" Size; Bronze Body & Spring Chamber; B2 Trim: Brass With Buna-N (Nbr) Seat,Neoprene (Bc) Diaphragm; Opt-31p: British Standard Parallel Pipe Thread End Connections; 80-150 PSIG Range Spring