A flat plate in (or opposite and below the mouth of) the pipe of an organ.
1973, Thomas Pynchon, Gravitys Rainbow'':
If there is music for this it’s windy strings and reed sections standing in bright shirt fronts and black ties all along the beach, a robed organist by the breakwater—itself broken, crusted with tides—whose languets and flues gather and shape the resident spooks here.