The Filter Locking Ring is a factory original replacement part designed for AquaPro above-ground pool cartridge and diatomaceous earth (D.E.) filters. It is compatible with AquaPro 100, 120, 150, 190, 200, and 250 cartridge filters, as well as D.E. 40 and D.E. 72 filters. Its primary function is to tightly secure the filter lid, maintaining a strong seal to prevent leaks and ensure the efficient operation of your pool's filtration system.