It turns out Edna was an American ship, originally the Mazatlan, seized for breaching neutrality - http://query.nytimes.com/mem/archive-fr ... 5B868DF1D3
I guess the implication is that the original German owner gave or sold it to an American company at the beginning of the war to stop it being confiscated.
It turns out the British had very good reason for suspicion - there is a lot of fake flags, coded radio messages, mysterious boardings of German naval vessels by night - all foiled by a heroic young British wireless operator named "Smith".
