This is an early model 7mm pinfire revolver manufactured in Liège, Belgium by Servais Joseph Celestin Mariette, the son of the well known gun maker, Guillaume Mariette.
This double-action-only revolver design had a screw at the bottom of the frame which when unscrewed allowed the front of the frame and barrel to slide off. You could then slide out the cylinder, load the cartridges, stick everything back on and re-screw the screw back on to tighten it back up.
It has a very unique sight attached to the top of the barrel that is not often seen.
You can see this in action in the following video.
You can read the full article on this revolver here: Mariette Patent 7mm Pinfire Revolvers