The American equivalent of the German Enigma, the U.S. Army Cipher Machine model M-209 allowed for the enciphering and deciphering of messages in the field. It was small, light, rugged and dependable and required no batteries.

Developed by Hagelin in Sweden for the US Army as the successor to the M-94 hand cipher used in World War I, the M-209 was used by the US Army during WW II and for a long time thereafter.

Text is enciphered by entered one letter at a time on the alphabet ring at the left side of the M-209 and turning the black knob on the right one full revolution. A coded letter is then printed on a paper tape. Deciphering requires setting a knob to the “Decode” position and entering the coded letters in the same way. The decoded message is then printed on the paper tape.

The cryptographic key includes the initial settings of the 6 rotors and the settings of the internal sliding clips on a drum. The rotors have different spacings of the letters on their rim. This adds complexity to the encoding process.

This M-209 is supplied with its canvas carrying bag, together with spare paper rolls, operator's manual and a message book. For simple maintenance of the machine in the field all of the necessary tools are stored inside the top lid of the case. At the right is a pair of tweezers, used for feeding the paper through the printer and removing blocked paper. At the left is a notched screwdriver used for setting the clips on the drum. Two small containers in the leftmost corners of the cover contain oil (labeled "O") and ink (labeled "I"). The top lid of the case acts as the paper tape holder with its paper guided directly to the printing mechanism on the left of the machine.