Insulator made of a clay like material. Possibly an experimental model from the Jubilee Line.

Glazed white insulator with central support. A smaller unglazed insulator has a hole through which the support is inserted. A metal sheath, which has "304 Cast No M7 18242" written on it in black is inserted over the clay support. The three parts are detachable. The whole device sits on 2 metal legs.