MS3509 Receptacles (7000 Series) are normally installed with an O-ring defined in the Milspec. Part number MS3509-20 (7002-5-20) is the same as MS3509-30 (7002-5-30), but it uses a "receptacle-shaped" gasket instead of the O-ring.

Rebling Plastics offers both a style 1 (100A1112) and style 2 (023A1169) gasket.

Style 1 gasket (100A1112)	Style 2 gasket (023A1169)