It wasn't just Martini's or low end guns it was guns of all types. Winfer’s BRITISH SINGLE SHOT RIFLES, VOL. 3; JEFFREY RIFLES JEFFERY & THE TRADE FARQUHARSONS WITH NOTES ON NITRO CARTRIDGES has the greatest detail about the trade.

You certainly can't call the Farquharson low end. That rifle type was widely made with parts from all over. The gun makers were interested in making money and they did it by getting the parts where ever they could.

Winfer also discusses how the guns were actually made. You may have a vision of a Gunsmith spending many hours sitting at bench in the shop at Holland and Holland making your fine Double or Farquharson. But in fact most of the work was subcontracted out to various different Gunsmiths and Gunshops. Thomas Turner made a lot of guns for the big names and especially Jeffery. You won't see Turners name on them anywhere except in Jefrrey's ledgers.

Winfer doesn't just say that is the way it is, he cites sources especially the gunmakers original records and makes quotes from trade publications of the era.

The practice of using parts from other sources and subcontractors was wide spread, and there is no reason to believe the double was excluded from such practice. Why should it.

I do strongly urge you to get Winfer's books and read them. He specifically addresses this issue of the stigma of Belgium guns and gun parts and the reality of the issue. They are well worth their price.