Regarding Captain Midnight and the so-called "decoder rings" of the era... up front: there were no "decoder rings" offered in the entire era of Old-Time radio. There were, though, "decoder pins."

The basic origin of "decoders" on Old-Time Radio was the Wander Company, makers of Ovaltine. It started with their radio program, Little Orphan Annie, in 1934. The sponsor started a fan club, Radio Orphan Annie's Secret Society. It had a membership pin and a member's handbook. Inside the handbook was a very simple ciphering scheme that any child could learn to decipher and encipher messages: each letter's position in the alphabet was given a number double its numerical position (i.e., A=2, B=4, C=6, ..., Y=50, Z=52, and &=54).

In 2000, Ovaltine offered the Ovaltine Secret Decoder Ring on the labels of its "tins." They advertised, "It's back!" despite that they'd never offered one before. We never had any because we could not afford Ovaltine.