Sta Rite, Filter Tank Replacement 21" O'Ring (STA1012821, 24850-0008) Fits: (Modular DE Mod: S7MD60 & S7MD72, System 3 DE, S7D75, System 3 Cartridge Mods: S7M120, S7D75, S7M120 , System 3 Sand Mods: S7S50 Sta Rite, Filter Tank Replacement O'Ring (STA1012821, 24850-0008) Fits: (Modular DE Mod: S7MD60 & S7MD72) & System 3 Sand Mods: S7S50 Fits: (System 3 DE, S7D75 ) Fits: (System 3 Cartridge Mods: S7M120, S7D75, S7M120