This postcard is of Herzog Ernst August (1887-1953) of Braunschweig. He became the son-in-law of Kaiser Wilhelm II (1859-1941) when he married the Kaiser’s only daughter, Princess Viktoria Luise (1892-1980). He is wearing the M-1915 Litewka of his Husaren-Regiment Nr 17. He is also clearly wearing that regiment’s schirmütze with the Totenkopf. The postcard was mailed in 1917.