I count at least 18 Thonons or hyphenated Thonons. The family extended from the 1830's (or earlier) to at least the 1960's. There was even a damascus pattern called thonon.

Go to the bottom of this page:
http://www.damascus-barrels.com/Image_Barrels_2.html

Which leads me to believe that some Thonon at some time patented this pattern.

Jules was around from 1908-1925. He was in business with Duchateau.

Fernand had a large operation, that began about 1903. 51 rue Monulphe Liege. How long he continued I am not sure. I do know that some of his guns were proofed in the late 1940's. I believe he was a maker and a dealer. I know he did business in Italy and Luxemburg.

Post some pictures of the markings and perhaps the gun can be dated.

Pete

http://www.damascus-barrels.com/Movie.html