The password that Hardt is to use for his contact is the opening line of Heinrich Heine's poem, written in 1824, "Die Lorelei," in which a beautiful woman lures sailors on the rocks.
This was the first film to pair Michael Powell and Emeric Pressburger, who went on to become an important partnership in British film history. Powell and Pressburger eventually made over 20 films together during the course of their partnership.
Although passed by the British Board of Film Censors on 10 March 1939 (with a "U" certificate) and reviewed in the Monthly Film Bulletin in March 1939, the film did not reach the cinemas until 3 August 1939. It opened at the Odeon, Leicester Square, and was doing quite well till its abrupt closure on 2 September 1939. When WWII was declared the British government closed all cinemas because of the nationwide blackout (which soon reopened during the 1939-40 "Phoney War").
There was a U-29 in the German Navy during the Great War. It was sunk in the Pentland Firth in 1915, by being rammed by HMS Dreadnought.