This is the only "correct" label I've ever seen for a cased Darne:

It is gold leafed and embossed. It is also a bit delicate. I took perhaps a dozen when I was in St Etienne, as the Paris office had been long closed, at that point, and nobody wanted them. I have seen paper labels that were reproduced for Darne guns, but, they aren't anything like this. It would be expensive to reproduce.
I've given a few of these away, over the years, and only have this one, left.
I would guess this label was used from the 1930s until sometime in the 1970s.
A factory cased Darne is tough to find. For the most part, the factory cases were Italian, and supplied from the Paris office.
Best,
Ted