Pressure filled glycerine 0-500 PSI, class 1.6 connection back ... 1/8"NPT, 2-3/4"dia.RW-RG-500Red pressure 0-500 PSI, class 2.5 connection bottom 1/8 ... "dia.ring for front mounting, class 2.5RW-FRG-500Red pressure 0-500 PSI, connection back 1/4"SAE, 2-3 ... /8"NPT, 2-3/4"dia, stainless steel frame presurre gauge