I would like some information regarding this stamp. For example, why the print is rotated and how much it could cost if I wanted to sell it. I would like to add that the stamp comes from my grandfather's collection.
@ MS Sammler [#2]
Caro Leon, unfortunately I don't have any certification, but I would like to have one. As I wrote, this stamp was in my grandfather's stamp collection, most of them were Polish stamps and among them there was a German stamp about which nobody can tell me anything.
@ Malgorzata [#3]
Fake overprints on the early Berlin issues are to be expected. If I have to judge the color and the impression, I would assume that you have overprint forgery. If you want help, a 600 or 1200 dpi scan would be very helpful, a blurred mobile phone image is not helping.
There are two Berlin experts at the BPP:
Andreas Schlegel and Hans-Dieter Schlegel 
If you need an expert opinion, please send the stamp to one of them. The costs are:
- Your registered letter
- Your insurance
- Invoice + VAT + registered letter (around 35 - 40 EUR) if overprint is fake
All the best,