Pentair Cyclone jet barrel diffuser - this is the white wine glass shaped plastic rear on the jet that often perishes with chemical abuse / mis-use - usually through the use of BROMINE.
It also has the locating lugs attached, which hold the jet barrel in the jet body. These lugs sometimes break off, requiring a replacement diffuser or jet barrel.
Suits all jet barrels in the cyclone range. 100mm long x 64mm diameter.