Nothing here that could be made into 'FN' and VERY unlikely to be Fabrique Nationale. It could be the dealer.
http://www.littlegun.info/arme%20francaise/a%20accueil%20arme%20francaise%20fr.htm

Assuming it is a light weight game gun, and you intend to use it for birds with 7/8 oz. or 1 oz. loads, leave the chambers alone. Measure shot 16g 2 3/4" hulls of various makers and you'll find some that are shorter. If the cones are (typically) 1/2", there might be some pressure benefit in lengthening the cones. Scroll down here about 1/3
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit
Modifying the chokes to fit your needs is certainly reasonable, and the gun could no doubt use a good check and internal clean before use.

https://sites.google.com/a/damascusknowledge.com/www/home