Janssen Sons & Co. was a Belgian (Liege) gunmaker from the early 1890s through approximately approximately 1931. The Damascus shotguns were imported early on, but importation stopped due to WW-I in 1914.
Because of the low grade steels in the guns, their value is very low - $100-300, depending on condition. Firing them is contraindicated, unless the shells are the original short (2 1/2- 2 5/8 inch) shells with black powder loads, and the barrels have been inspected by a gunsmith.
Basically, they're mantlepiece display weapons.