What is "code-o-graph"

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.