The difference is in who wrote the ad copy. And you put your finger on the issue- the GUN itself was never intended for smokeless powder. Get a nice set of mounts, and it will look great above the fireplace.
Janssen & Sons was a "Trade Brand Name" used on guns made by Janssen Fils & Company, a Belgian gun maker located in Liege ,Belgium from 1893 to 1957. They had two addresses, 42 Rue Fusch 1893 to 1939 and 29 Rue Duvier 1946 to 1957, I guess that they were closed down during World War II. Guns with this name were made for export to the US. I have no information as to whom the importer or seller was.
What we have here is... failure- to communicate.