Hi WesF and Welcome!
That is a Belgian double you have there, and the letter f on the barrel flats suggests a proof date of 1927.
Looks like a very nice rifle, thanks for showing us.
Regarding the "A Thonon" on the barrels, I cannot offer much help other than this link to a site with some info on Belgian gunmakers including the Thonon name.
http://www.littlegun.be/arme%20belge/a%20a%20artisans%20identifies%20gb.htm

Would love to see more photos if you get a chance.
I own and use a Belgian .470 proofed in 1928.

Edited to add..."Namur" would most likely be the town in Belgium where the A Thonon was located.