This filter will fit most Softubs hot tubs pre 2000. Pleatco PTL20W - SV, Unicel: 6CH-25. Softub P/N 90-802.

Softub Screw on filter
Filtration Size: 20 sq ft
Outside diameter: 15.2cm
Top finish: Handle
Bottom finish: Fine male thread (3.8cm)
Length: 14cm