Hello Marc,
From this Point of view I would say that`s not a stamp.
That are scratches,deep scratches.
Try to make better Pictures ,closer and with different light.
I had the same Problem with a Mauser Pistol.
It Looks like scratches if you have the pistol in your Hands and check the stamp only with your eyes.
I used the macro function on my camera and different light.
The " scratched aera" is a bad stamped tiny eagle with swastika .
Thanks
Stefan