This older answer should be of interest.
Also, how much before, can its fashion be separated from Hitler? In Herman Wouk's The Winds of War / War And Remembrance books, a character refers to you-know-who as "the villain in the mustache" in 1939 before the war breaks out (but in a book published 1971.) The author's research and details are generally very good for a historical novel, but is this a slight anachronism?