[dbaron]

Now that I'm home: the scan is from page 508; the relevant bits are from the end of page 504 through page 508, or perhaps all of Chapter 67 (502-509).

Personally, I'm stumped by the "without anything passing in writing" part, which seems to me to make the problem unnecessarily harder, since Pandarus was carrying codes for them printed on silk (to be sewn into the lining of clothing).

And... since I haven't mentioned it already: this is a great book. It's gripping, and reads a little bit like a movie for a good reason -- its author (who is telling the story of his experiences working on cryptography during the war in his early 20s) went on to become a screenwriter.

[rottendoubt]

Thanks for the clarification. So it has to be some sort of verbally transmitted code (ie. a phrase) since it's used over the radio? That pretty much takes out almost all the guesses posted here so far.