[Jim K]

I have never seen anything quite like it. If it is Belgian, then it should show the "ELG" proof mark on the frame and cylinder. Another possibility is Spain, which had no effective proof law before the early 20th century and whose makers often counterfeited other nations' proof marks. It is too well made to be a "Darra" gun or something like that.

Closeups of the proof marks would help.

FWIW, It looks almost like the (American style) latch was from another gun and screwed onto this one. In fact, it has the look of a one-off experimental or model, made from parts of one or more existing guns.* The frame around the hammer (with that odd "dip") looks like it was machined into the frame after the frame was made rather than as part of a production frame, and the lower barrel lug has the look of a part welded on or altered.

*Which might account for proof marked parts but without the normal marks in the usual places.

Jim