Re: Defourny and Monard
That company was in business late enough that the barrels should be solid steel and OK for smokeless powder shells. But the chambers are almost certainly cut for 2 1/2" shells (the proof marks will give the shell length in millimeters, 63 would be 2 1/2").
Those guns were among the best of the Liege guns and can run into good money, with boxlocks reaching the $4000 range and side locks going even higher. I would recommend a professional appraisal. I don't know about others, but I don't feel knowledgeable enough to give a valid appraisal even if you had provided good pictures.