9500-52MM

A bonded seal combines a metal washer with a rubber sealing element that's vulcanized directly onto it. When you torque it down, that elastomeric ring compresses against the machined port surface and forms a leak-resistant face seal. A plain washer doesn't have that sealing element, so it won't close off fluid paths the same way. The bonded construction is what makes it work as both a fastening and sealing component at the same time.

For a 52mm setup, this bonded seal is meant to sit between the bolt head and the machined port face. When you torque the bolt down, the metal washer supports the seal and the vulcanized elastomer compresses to make a leak-resistant face seal.