A radio premium, given away during the golden age of radio drama, to children who sent in enough boxtops. The badge would usually display the name of the show, such as Captain Midnight's Secret Squadron or Radio's Orphan Annie.
At the end of a show, a string of numbers would be read out and listeners could use their secret decoder badge to decode the message, which was usually advertising from the show's sponsor, for example, "Drink Ovaltine". Some badges had a fixed dial and so could only decode 1 cipher but others had a moveable dial which increased the complexity by an order of magnitude.
I have a 1941 Captain Midnight Secret Decoder Badge and that makes me cool