Barbie typewriters, sold worldwide by Mattel, have an undocumented built-in cryptographic capability. Pressing SHIFT and LOCK in combination with a particular trio of keys will engage any of four monoalphabetic substitution ciphers — once the feature is engaged, a keyed message will be printed in a transposed alphabet. Pressing a different combination of keys will put the machine into “decoding mode,” where keying a transposed message will print the deciphered text. So the same machine can be used to code and decode a message.
Details are at the Crypto Museum. These things were marketed to 5-year-olds. What else don’t we know about?