Davey Spa-Quip in line Heater Temp Sensor.
Fits the following models, SP400, 54500, 500A, 500AMK2, SP600, SP601, SP800, SP1200.
Spaquip Spa Power 400, 500, 600, 800, 1200 In-Line temperature sensor - fitted in the heater element assembly.
NB - Suits the old style element pictured in the photo.
Does NOT suit newer Titanium heaters fitted to Spa Power 601 and Spa Power 800 controllers since late 2015