The KY-171 Audio Frequency Coder was used for the NASA Project Mercury (1958-1963) to encode radio traffic between ground stations, as well as between ground stations and the space capsule. It was used with the T-560B Radio Transmitter to send encoded telemetry data. The R-669A Radio Receiver was paired with a KY-172 Audio Frequency Decoder and the C-1669 Coder Control to decipher the messages. All of these devices were part of the AN/FRW-2 Radio Set.
Now… I have been looking around the internet for weeks now, but I can not seem to find any specification or paper describing the encryption/decryption algorithms/techniques used. The only thing I've been able to dig up is the schematic, which is printed on the inside of the bottom plate of the coder itself.
Can anyone confirm if any written documentation related to the used crypto is available to the general public at all? If it is, where can I find relevant papers describing the cryptographic algorithm(s) and/or techniques used within KY-171 and KY-172?