Right, that is why I suggested it could be an earlier cornet of his.
Looking at it more, it doesn't look like a French instrument. Those style of keys are very much the German style with the shape of the key and the large "puck" the key is attached to. Besson did make some rotary valve cornets (e.g. Robb has one on his website) but the keys don't look like that. The thin bracing also does not look like any Besson bracing I have seen. Also that style of engraving looks later to me, the French engraving of that period is pure lines; that style of engraving is what was popular post-1900. And I agree the engraving looks too sloppy for a Besson. The water key has a coil spring on it which is also post-1900.
There could be explanations for this, e.g. Besson was copying the recent German designs and had not yet added their own touches to it. But, I would say some healthy skepticism is in order until there are more details 😁
PS Here is a German cornet from around 1900, it looks a lot more like this than like any French instrument (notice the bracing for example). Also this style was patented in 1900 and popular then .. more strikes.