I have no experience with this item, but there are two notches visible on the inner ring. You would need to turn that ring counter-clockwise to remove the freewheel. Rather than a special tool, I think you would just tap against the notches with a screwdriver or flat punch and a hammer. (In a recent thread, someone succeeded with that method.) The two dimples on the central ring are for disassembly of the FW rather than removal.

Incidentally, I own a German-made air rifle made by Dianawerke. Eine ausgezeichnete Waffe.