[ThePadawan]

The ancient Greeks beat this in terms of slow delivery:

"In 499 BC, he shaved the head of his most trusted slave, tattooed a message on his head, and then waited for his hair to grow back. The slave was then sent to Aristagoras, who was instructed to shave the slave's head again and read the message, which told him to revolt against the Persians"

https://en.wikipedia.org/wiki/Histiaeus

[ncmncm]

Need a stable of slaves with different tattoos. Or maybe just two: revolt, or wait for next round. Or put a numbered list on one, and send the next with just a number.