In the 1950s Ovaltine sent out Captain Midnight decoders. Somebody bought one at an antique shop and solved it.
I had one as a kid, naturally.
https://www.youtube.com/watch?v=WvKlqMjfk1Y
Note that LaurieWired is very cute...
A local TV station in Memphis when I was a kid in the 60’s had a ‘club’ you could join and they sent out card that had a code decoder on the back, and every day after school they would have the kids ‘solve’ a secret message during the cartoon show.
Then they would draw your phone number out of a fishbowl and call you. If you had the secret word correct for that day you would win some prize like a model car for boys or doll if you were a girl.
I had the decoder card, but we didn’t have a telephone and it would have been long distance for them to call since I was in Mississippi..............😪
When I was about 12, I got a book on codes from the library - I only remember the simplest one that used a tic-tac-toe grid and a big X as the basis.