You're right and I agree with you OWD but it seems that workers who supplied guns 'in the rough' would not or could not or, more importantly, were not trained to, 'finish' the gun whereas the London makers took the rough stock and because they were trained in finishing brought the rough piece to its final complete form.
That's just speculation on my part but that seems reasonable. Referring back to Holland & Holland, prior to building their factory it would seem they took forgings that had been rough out into shape and with their workers finished the gun to its highest standard. These rough forgings would be almost impossible to track down or why would it matter who roughed out the forging.