800-001-6 / 800-0001-6 Cessna lock-o-seal. The Lock-o-seal incorporates an o-ring that sits inside a washer but is not attached. This washer seal combo is used in several Cessna brake master cylinder assemblies. The 0861710-5, A-049-7, the 0541138 series, 9882010 series and the 5044000-4.
