Cypher Machines
A plugboard was used on the famous Enigma machine; it was not removable. In this case the plugboard acted as a "fourth rotor" in the rotor machine's workings. Plugboard wirings were part of the "day settings" that specified which rotors to insert into which slot, and which plugboard connections to make. In practice the plugboard did improve the security of the cypher being generated, but as it did not change with every keypress, unlike the rotors, its impact was limited. See Cryptanalysis of the Enigma.
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