It is a proof mark, but English (Birmingham), not German.
Wirnsberger shows the mark in his table and says, "In use from 1868-1925, this mark was used to indicate a single, final proof with loads designed for temporary or provisional proof."
He also shows the mark in the text, saying that "On special application, a smoothbored gun could be proofed but once, but in accordance with the load set forth for final or definitive proof."
That would seem to indicate that the gun is smoothbore. Is it? If so, and considering the "special application", I wonder if it could be a gun that was used in some show or other. Smooth bore rifles (a contradiction, but you all know what I mean) were often used in "Wild West" shows so trick shots could break balloons or flying targets without firing bullets, which would not only be dangerous but would tear up a tent, which small "dust shot" would not.
Jim