Dead Drop from the 1870s
Schneier on Security 2013-03-15
Summary:
Hats:
De Blowitz was staying at the Kaiserhof. Each day his confederate went there for lunch and dinner. The two never acknowledged one another, but they hung their hats on neighboring pegs. At the end of the meal the confederate departed with de Blowitz's hat, and de Blowitz innocently took the confederate's. The communications were hidden in the hat's lining.