This was done in France before WWII as well.
There is such a gun for sale right now in the US. Of course, I don't think that the seller has any idea. The gun is way overpriced, or I would have bought it for the heck of it.
If you see a French gun with "Monobloc" on the barrel flats, it's a one piece barrel set.
I think that Jean Breuil, the famous barrel maker from St-Etienne, was a specialist.
It's still quite rare. I only have heard of a few. One was on this forum at least 5 years back.
This is not to be confused with "canons frettés" which is the same as the US "Monoblock". The Robust is a famous example of such a gun.
Best regards,
WC-
