The Code-O-Graph is a field cipher device and identifier from the Captain Midnight radio serial. In the story line they were used by agents of the Secret Squadron, a paramilitary organization headed by Captain Midnight. In addition to their use as devices in the radio program, they were offered as radio premiums for listeners. As radio premiums, listeners sent in an Ovaltine proof of purchase with their names and addresses. The Code-O-Graph incorporates a cipher disk, a cryptological device, but the letters in the message were substituted by numbers, probably to avoid ambiguity for broadcast messages. On the radio show, usually once a week, a message would be sent that could be deciphered by listeners. Each message, only a few words long, was invariably a clue to the upcoming episode. Models The first Code-O-Graph, called the "Mystery Dial" unit, was introduced in 1941, as a device to enable Secret Squadron agents in the field to send and receive secure messages. It was in badge form, as were the next three models. The front of the badge displayed the number and cipher alphabet scales. The reverse had two windows, one labeled "Master Code"; the other, "Super Code." each window was used for cipher key settings. As an example, if the cipher was designated as "Master Code 3," it meant that the movable rotor was to spun so that the number 3 would appear in the window labeled "Master Code." This setting would align the number and cipher alphabet scales correctly to decipher a message. The second Code-O-Graph was the "Photo-Matic" unit. The badge had a space for a picture of the owner, to make it a photo-ID badge. The advent of World War II affected the Code-O-Graph availability: the two previous models were made of brass, and the attack on Pearl Harbor, which propelled the United States into World War II, caused the U.S. Government to impose restrictions on manufacturing materials. Copper and brass were considered critical materials, and most of the materials were diverted to war activities. This precluded brass being used to manufacture novelties like radio premiums. The Photo-Matic Code-O-Graph, although not distributed until 1942, was manufactured prior to the Pearl Harbor attack. Since it, and its predecessor, were undated, the newer Code-O-Graph was used for the 1943 and 1944 seasons as well as the 1942, making it the Code-O-Graph with the longest service life. The cipher setting scheme