Locking Ring for Trimline Cartridge Filter Lid MK2 MK3Take control of your Waterco Trimline cartridge filter lid with the Locking Ring MK2 MK3 a purpose built replacement that secures the lid, reduces wear, and makes maintenance smoother, safer, and less prone to damage. Designed specifically for Waterco Trimline MK2 and MK3 lids, this locking ring is the essential upgrade when a lid sticks or when you want to avoid costly breakages from prying with improvised tools. Its the part you install once and