Germany Belgium Occupation WW1 Prufungsstempel

Unable to find these listed in catalogues; seeking information.

In my opinion the "cancellations" are fake. Everything seems to be wrong about them; looks like water-based ink and a rubber stamp.
They do seem to be trying to look like something similar which is genuine. See the American Philatelist article "Censorship Markings: "GEN.-GOUV. WARSCHAU," 1915-1918, Part II in the December 1984 issue. (Part I is in the May 1984 issue.)

genuine marking from the article:

Langensalza (Thuringia) was a PoW camp during WWI and a concentration camp during WWII. I wonder how the Generalgouvernement Warschau has any relation to it other then mail from a prisoner sent there.

Here is a similar example.
Postüberwachung: Post monitoring

Your Postüberwachungsstelle postmark looks good.
The German website stampsx has a thread about this postmark but not too much information. In German, of course. If you can't see the images you will have to register (no cost).

https://www.stampsx.com/forum/topic.php?id=14393&

https://www.stampsx.com replies;

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