Spa Power SP54500 “Fine Thread” Water Sensor “9.22mm OD”.This Q915440 is the fine thread pitch, long water sensor for the SP54500 and very early SP500A MKI (light blue sticker with reset button)
These are obsolete and no longer available. Upgraded Controller Available Here.
This listing is only for SP54500 (Fine 9mm thread). The later 11mm version is listed here.