You are purchasing one CX900F O-ring. Superior material for your Star-Clear Plus Cartridge Filter Head. This is what you want. Product fits perfectly and is of very high quality.
Full List of Compatible Hayward Star-Clear Plus Filters for CX900F Filter Head O-Ring: C1200, C12002, C12002S [C12002(S)], C1502, C17502, C17502S [C17502(S)], C2002, C751, C7512, C900, C9002, C9002S [C9002(S)]
Mallard Armory product packaging includes a Mallard Armory manufactured o-ring doubled over in bag. For your pool equipment, trust only Mallard Armory brand seals.