Thirty-odd years ago I read Alvin’s Secret Code by Clifford B. Hicks (1963), which begins when a puzzling telegram falls into the hands of two boys. The only sentence in it that makes any kind of sense to them is “Ivan hiding message oak,” so they look in a hollow oak tree but find nothing. They take the telegram to a retired spy, who explains that it is in a commercial code which, as luck would have it, he devised; “Ivan hiding message oak” means “Jones arriving Blanksville Wednesday.”
Now I learn from Kahn’s The Code-Breakers that, in the jargon of the Russian Communist underground, dubok ‘little oak’ meant a hiding-place for messages. Hmmm.