Original W.W.II period german "Feldpost". It has a rare stamp with open map reference which is "Marine-Bordflakbrigade Nord" (Hamburg, Stettin, Oslo and Rotterdam).

Text on front:
"Verwaltungsausschuss der Offz.-kleiderkasse der Kriegsmarine, Wilhelmshaven"

translation:
Administrative committee of the Officer's clothing department of the german navy, Wilhelmshaven