My take is that Captain Elwood sent him a letter more or less reading "This is a Morse code recording from a domestic station but we won't say more about it, bu

My take is that Captain Elwood sent him a letter more or less reading "This is a Morse code recording from a domestic station but we won't say more about it, but it's not aliens" and that only riled Davidson up, presumably because he wasn't talking about aliens in the first place, so why the red herring. Now they ask to find the decoded message and give the man the sending station so he shuts up. At the end they warn that just sending him some no-content letter (or [I am guessing they imply] made-up stuff) won't do.