Ovaltine offered cryptological radio premiums, called Code-O-Graphs. These were used to identify the bearer as a member of Captain Midnight's Secret Squadron. Their primary use was to decipher "Secret Squadron Signal Sessions," messages sent at least once a week, and invariably as previews to the following episode.
This is the Mystery Dial Code-O-Graph, the first of seven Code-O-Graphs. It was so called because the knob on its front was supposed to resemble the tuning knob of a contemporary radio.