Here’s a trip down memory lane for some of you, particularly those of you who served in the RFA back in the day. No doubt you will remember the KL-7 crypto machine. What a beast that was…
If you’d like to revisit the KL-7 you’ll find an excellent simulator on the Cipher Machines and Cryptology website build by Dirk Rijmenants, here: https://www.ciphermachinesandcryptology.com/ He’s made the simulator as true to life as possible, complete with the sound of the machine’s motor-generator and the hearty clunking noise of the rotors turning with each key press. The only thing the simulator doesn’t do that the real machine often did is to stop working in the middle of a long decrypt due to dirty contacts under the keyboard!
There’s a lot more fascinating stuff on the site if you’re interested in the history, technical and operational aspects of cryptology, including a section on the Enigma machine (an excellent simulator is available for that, too) and several other mechanical and hand-cipher systems.