This is the secure voice encoder/decoder we used during the last 3 or 4 months I was in-country. Since I was the only member of our team with a secret clearance and was an E-5, I had to sign for the unit. I chained it to our radar van.
We punched in new codes every night at 24:00 hours (midnight). A week’s worth of codes were delivered to us via the duty helicopter once a week. Either our section chief or I carried the codes on a string around our neck. The section chief and I met in the morning and burned the previous night’s codes.
These secure voice rigs worked so well that we could transmit any information desired such as names, our location and of course, our detected target information.
Missing from the picture is the pistol-grip cartridge that contained the adjustable pins that set the variable code switches inside the case. As you can see, there were lots of pins and each one of those pins had a dozen or more possible settings, making for probably millions of possible combinations. In other words, even if the enemy possessed a KY-38, it would’ve been useless to them without a current code book.