This Nema Machine was classified and used primarily by the Swiss Army and Air Forceup until 1992. …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.