This Nema Machine was classified up until 1992, it was used primarily by the Swiss Army and Air Force. Video demonstration after the jump.

…the letter you have typed is immediately encrypted into another letter – a letter will light up on the light board, which is above the keyboard. However the electrical circuit that is created to cause the light to go on passes through a system of rotors, and after every key stroke, at least one of the rotors rotates, so if you were to press the same letter a second time, it would now encrypt as a different letter

This Nema Machine was classified up until 1992, it was used primarily by the Swiss Army and Air Force. Video demonstration after the jump.

…the letter you have typed is immediately encrypted into another letter – a letter will light up on the light board, which is above the keyboard. However the electrical circuit that is created to cause the light to go on passes through a system of rotors, and after every key stroke, at least one of the rotors rotates, so if you were to press the same letter a second time, it would now encrypt as a different letter