Pick up Wal Winfer's on British single shots. He talks about the process at length.

The Key person behind it was Auguste Francotte. He would acquire the parts from various cottage parts makers in Belgium, import them to England where the English Gunmakers bought the parts and finished the Guns out in their shops. The only Belgium Provenance might be a single letter or a short number on the action body that didn't match anything else on the Gun. The British Gun makers applied their serial number, view and of course Proof marks. Born in Belgium, citizen of England!!

The trade Farquharsen’s were almost all done this way, according to Winfer, as were some Martini's.

There is no reason to believe the practice wasn't applied to Doubles as well.