Klek was never occupied by german troops in WW1 - only at WW2!
it was part of austrian-hungarian Empire from 1867 - 1918.
1918 - 1941: Klek became a part of the Kingdom of Serbs, Croats, and Slovenes (this would become Kingdom of Yugoslavia after 1929)
http://en.wikipedia.org/wiki/Klek_%28Zrenjanin%29
http://www.im-banat.eu/

Vojvodina - also read for the time-period 1941-1945 at "Serbian and Yugoslav administration"
http://en.wikipedia.org/wiki/Vojvodina

"and there is a stamp at the bottom which has the German Eagle"
can you load up the document in german for taking a look on it and seeing the "german eagle" and what is written?