Tecmark Model 3903 Pressure Switch , 1Amp power rating 1/8MPT screw thread to attach to heater tubes, fitted with 2 x quick connect terminals for the electrical connection
1-5 psi 7-35kpa adjustable SPST.
this is a very common Pressure switch, may be used in the following heaters:
Balboa , Davey , Spa-Quip , Spa Builbers , HydroQuip , and other