>Man in the Box
The origins of this encoding go back to the Polybius square of Ancient Greece. Like the "knock code", a Cyrillic script version is said to have been used by nihilist prisoners of the Russian czars.
https://en.wikipedia.org/wiki/Polybius_square
Polybius checkerboard, is a device invented by the ancient Greeks Cleoxenus and Democleitus, and made famous by the historian and scholar Polybius. The device is used for fractionating plaintext characters so that they can be represented by a smaller set of symbols, which is useful for telegraphy, steganography, and cryptography. The device was originally used for fire signalling, allowing for the coded transmission of any message.