Fits most recent Raypak (by Rheem) gas pool and spa heaters from the last decade or so. Originally grey, then grey with black label. Now are only black. 

Not sold as a spare part by Raypak Rheem, but we sometimes have these in stock anyway. 

The PCB Circuit Board / Thermostat to suit is here:
https://melbournespaparts.com.au/products/raypak-gas-heater-thermostat-circuit-board-pcb