Chris,
This is a very interesting mark of the Schutzpolizei or possibly Sicherheitspolizei of the Prussian administrative district of Arnsberg. The vertical slash is very curious but at least one other example exists on this Imperial-era tool:
I am writing a book on German police markings and would greatly appreciate your permission to include your photo of the grip strap. I will, of course, credit you. Thanks very much.