This is the 4-Rotor Naval Enigma machine employed by Germany's U-Boats in World War II. The addition of the 4th rotor which did not turn added an additional base-26 order of magnitude to the complexity of the machine's cipher. In addition to adding the 4th rotor, the machine came standard with a 7 rotor collection to choose your key from instead of the 5 rotor complement the army's standard 3-rotor machine came with. Additionally, the Naval machine came with a different reflector known as THIN B, its internal wirings were different than the THICK B reflector used in the standard 3-rotor machine. However, when the 4th rotor was in a particular setting, the 4-rotor naval machine perfectly emulated a standard 3-rotor Army Enigma machine making communication between the different military branches possible..