Chuck: The Culper Ring used to use stuff like that to pass along information during the Revolutionary War days.
The Culper Ring was a spy ring organized by Benjamin Tallmadge under the orders of General George Washington in the summer of 1778. The Ring's task was to infiltrate British-controlled New York City and report troop dispositions and intentions. The Ring conducted covert operations until the end of the American Revolutionary War, though its heyday was between 1778 and 1781. A modern day Culper Ring has also been depicted in the comic book "Y The Last Man".