Poolmaster 12721 Replacement Filter Cartridge for Sta-RiteTX 50 WC108-56S2X Filter The corresponding Pleatco number is PSR50-4 and the corresponding Unicel number is UHD-SR50 The top of the replacement filter cartridge is open 4-3/16 and the bottom is open 4-3/16 with 12 concetric slots Break-resistant bands keep pleats separated for greater flow, filtration and add longer life to cartridge High quality spun bonded polyester material for exceptional water filtration Premium grade polyurethane and plastisol end caps