Original WWII German envelope (cover) for an Army Postal Service (Feldpost) letter. No letter inside, nice ink stamps, Lublin, Poland.
Unsure of date so the Feldpost number may be attributed to 5. Kompanie Infanterie-Regiment 14 (Mobilmachung-1.1.1940), Fp.Nr. 18642 B (2.1.1940-27.4.1940), or  Stab u. 1.-4. Kompanie Landesschutzen-Bataillon 614  (1.2.1941-11.7.1941).