The replacement Silicone Seals for the Flexifit HC432 are directly reverse compatible with all but the earliest Flexifit HC431 Full Face Mask. These silicone seals come 1 per package and in your choice of 3 separate sizes. Fisher & Paykel originally designed the HC431 to include 2 Silicone seals; sizes Small and Standard now it includes 3 seal sizes; Small which were carried over to the HC432. The size added was a smaller size so the original Small became the Medium, and the Standard became the Large. The HC432 only incudes 1 seal size.We have had customers mix seal and cushion sizes for a better fit.