These Foam Diffusers are designed for use with the Diffuser Elbow available for all FlexiFit 405 (HC405), FlexiFit 406 (HC406), and FlexiFit (HC407) Nasal CPAP/BiPAP masks; and all Oracle (HC451 & HC452) Oral CPAP/BiPAP Masks. Each pack includes ten foam diffusers. Fisher & Paykel - Part 900HC439