https://www.intheswim.com/p/system-3-s8m500-small-cartridge-replacement-25021-0224s/613895.html
Sta-Rite Small Cartridge for System 3 S7M400/S8M500 - 25021-0224S | In The Swim
The 25021-0224S Replacement Small Filter Cartridge is compatible with the Sta-Rite System 3 S8M500 Modular Media Filter.