As noone else tried to answer: A "Ch.Darlin" is completely unknown to us and, as you wrote, doesn't appear in any file. He may have been an otherwise unknown country gunsmith who retailed the gun.
But I suggest you recheck the spelling of the name and the proofmarks, if they are Belgian at all. Reason: In the French gunmaking center St.Etienne there were three gunmakers named "Charlin". Though the Charlin name is best known for sliding breech shotguns similar to the Darne ones, one of them may have produced conventional guns also.