A fitting including an electrode assembly mounted and sealed therein. The fitting configured to be positioned within an aperture in a wall of a socket of a prosthetic apparatus. The prosthetic apparatus may include an external socket that receives and is attached to the socket as an internal socket. The electrode assembly has one or more skin contacts for conduction of myoelectrical signals to a prosthetic system. The fitting includes a groove and a retention rim to secure and seal the fitting within the aperture in the socket wall, in order to prevent the passage of air and moisture from the interior of the socket when a stump of a limb is received therein. A unidirectional valve provides for the release of air from the interior of the socket.