drakegoodman posted a photo:
Letter on reverse (below) authored on 19.8.1917 and addressed to a Carl Heller in the military hospital 124 in Weisbaden. This letter was sent by a civilian, hence the requirement to add a postage stamp.
Thanks to our resident expert Yavor Dinkov, the aircraft in question has been identified as a Siemens Schuckert E type.
The Siemens-Schuckert monoplane first flew in late 1915. It was constructed with wire braced wings, a Siemens-Halske Sh.I rotary engine and was armed with a single LMG 08/15 machine gun. After a successful evaluation, just 20 production models were built.