St Magnus (the name of the steamer sailing from Longhope for St Romness and Thurso during the closing stages of the film) is the Patron Saint of Orkney. There have been several ships in that area with the name St Magnus.
The film begins on Saturday March 17th 1917.
The password that Hardt is to use for his contact is the opening line of Heine's poem Die Lorelei, in which a beautiful woman who lures sailors on the rocks is used as a symbol of homesickness. When his crew joke about him spouting love poetry to a woman in the dark, they miss half of the point.
There was a U29 in the German Navy during the Great War. It was sunk in the Pentland Firth in 1915, by being rammed by HMS Dreadnought.