Ah, OK, it was tough to see and I couldn't make it out.
That appears to be a Belgian proof mark, used in Liege.
According to some sources I'm finding online, it's been in use since 1852 and is used for rifled firearms proofed for smokeless powder...
There is a similar British proof from the same time frame, but it's for use on rifles, and the hammer style and lockwork (from what I can tell from the pictures) screams continential Europe.
The lock system appears to be based on the Chamelot-Delvigne design, which when it was developed in the late 1860s/early 1870s, was the best lockwork design in the world.