Prussian police unit marks can be found here:
http://oldmilitarymarkings.com/police_unit_marks.html
Dusseldorf?
The barrel would have most likely been completely replaced by a new 9mm one, rather than re-bored.
Make sure you remove the red oxide rust from the grip. Use well oiled 0000 steel wool on a well oiled rust surface and be gentle and patient.
Marc
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.