Nos fusils= our guns
poudres pyroxilees= nitro powders

As already mentioned, it is a 12 bore with 65mm chambers i.e 2.5".
This gun was proved for:
3.46grams "T" nitro powder and 38 grams shot ( If I read right)
The normal charge (i.e what to use for hunting) was
5.25grams of #2 Black powder and 32 grams shot (ditto)
This is a turn of the century (the 20th)lower grade shotgun. Rebounding Hammer back action with top lever opening.
Nice piece of wood.

The closest thing I found in the 1900 catalog is the 655 grade
which sold for about 200FF, i.e. the same as the lowest grade "Ideal" model.
As far as value, the other posts are about right IMHO.
Please remove the rust, it hurts to look at it this way.
Was your grandfather in Europe during WWII and liberated that gun?

Best regards,
WC