Thank you Researcher. The dealer was at least recognizing the 1900 Olympic games with the gold "Grand Prix Paris 1900" on the barrels, and possibly the gun was made for a lady Pigeon shooter??
The original bore 15.8mm = .622"
Right 15.5mm = .610" or Modified
Left 15.2mm = .598" or Full
Léon de Lunden of Belgium; winner of the once and only demonstration sport of Live Pigeon shooting at the 1900 Paris Olympics;
Grand Prix de Exposition Universalle de 1900. Appears to be using a hammer gun.
More information here
https://docs.google.com/document/pub?id=185YOyQl7GIB9OYLs9Hr3tnMLHqs4rjEdR4j_E9l4HLw 
