Félécitations, Grand Veneur,
You found a very rare rifle by one of the greatest gunmakers in history (now sadly defunct). From what I can see it's a working-grade rifle that's been correctly restored. Francotte was not the only Belgium maker to offer 600NE rifles to the public, but one can be sure only a handfull were made. I can strongly recommend the recent re-edition of a classic book (1931) by F. Courally, of Lebeau-Courally, ("Les armes de chasse et leur tir")for more information on double guns in general and Belgium gunmaking before WWII in particular.
Greetings,
Livinus