Hal, I believe the following screw has to be removed to release your barrels from the action (I suggest picking out as much grunge as I could from that slot before trying):
[Linked Image from i.imgur.com]

Many Lefaucheux-type actions incorporate a small lever on the fore-end piece to more easily remove the barrels (an invention of the Parisian maker/inventor Jean Lepage), but yours does not have it. Once the barrels are off, the various proof marks should then be visible, and barrel maker's marks, if any.

The screws/pins on your gun seem to be in very good condition!