Offenbach is on the Main (pronounced "mine") river, and its full name is Offenbach am Main or Offenbach on the Main. The holster marking is the abbreviated form "OFFENBACH A/M" with the second "A" (for "am") as a sort of superscript form above the slant sign.
The name written that way would normally mean there is another Offenbach, but I don't know of any.
Jim
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