Daryl, the French had a bunch of barrelmakers. Even today, it's my understanding that Verney-Carron makes barrels for other companies. In fact, that odd-looking "Metro" barrel marketed by Hastings, and which (along with low velocity loads) will significantly reduce the sound of a shotgun report, is made by V-C.

As far as acceptance of Belgian proofs, the European countries seem to have gone back and forth on accepting each other's proofs, until the CIP was formed. But from everything I've read, they all seem to have tried to bring their own proof laws into sync with other Euro countries, even pre-CIP. But France did retain a far stouter optional proof than just about anyone else.