An early example, this is made from two wooden parts with brass contacts.
The body is in two parts, both made from wood.
The side pins are metal, probably steel as these have surface rust.
The two contacts are brass, with threaded posts and hexagonal brass nuts for the two wires.
A vertical separator between the terminals keeps any strands of wire away from the other side, preventing a short circuit.
This would have been used with twin twisted flex, rubber insulated with cotton or silk braid.
This video features this and several other wooden plugs.