Reproduction case labels are available, they've been available for generations.

The case for my Mahillon two barrel set has without doubt contained my Mahillon and it's kit through the twentieth century, the label in the case has been present with the firearm affixed to the inside of the lid.

The label is a Greener label though, and Graeme Greener notes that the case (and my rifle too for that matter) aren't Greener products.

Might my gun have been brokered through Greener at some point, perhaps through the Paris shop some time in the late 19th Century?
Who knows?
I've done quite a bit of writing and calling around the world to many who'd be some authority on the subject, and have come up with little to no results on that particular issue with my rifle's kit.

This link will get you to Alain's work on my gun via his site, Littlegun. A label, even one that's been with the gun in a case for generations isn't proof of who made it or who it was made for, nor does it necessarily prove that the case is original, and that doesn't need to be such a bad thing. In the case of my Mahillon, it just makes things more interesting to me.
http://littlegun.be/arme%20belge/artisans%20identifies%20m%20o/a%20mahillon%20gb.htm

--Tinker