Letterlocking
Letterlocking is a lost art of protecting letters from surriptitious readers.
The Spiral-Locked Letters of Elizabeth I and Mary, Queen of Scots is a new article in the Electronic British Library Journal, documenting and explaining letterlocking, "the process by which a flat writing substrate is turned into its own envelope – is a system of document security used to send letters for centuries before the invention of the mass-produced gummed envelope in the early nineteenth century." These techniques clearly show an evolution of attack and defense over many, many years.
There are some fascinating elements to the design, which incorporates both manipulating the paper and a wax seal covering it. The authors are concerned with elements of preservation and history. Their distinction of high/medium/low security are relative to what's available within letterlocking techniques. Having spent time with tamper evidence enthusiasts, I think I can see how to disassemble and re-assemble a lock, but that's not the point.
The available technology to protect letters, especially between mutually distrustful parties, didn't include cryptography, because what we now call cryptographic and stenographic techniques were closely held secrets.
For more, Letterlocking.org. The CIA Flaps and Seals manual doesn't seem to mention letterlocking (although I didn't review closely).