Complete Harwil Flow Switch Tee, very common and used on a range of spa brands with low flow laing heating systems.
Long probe version, Length of grey probe arm: 46mm (1-7/8"), Outer thread diameter: 20mm (3/4"), Total Length: 90mm.
Short probe version, Length of grey probe arm: 35mm (1-3/8"), Outer thread diameter: 20mm (3/4"), Total Length: 80mm.