I wonder if the turbulent times during which some of these civilian firearms were assembled had anything at all to do with the the re-use of older components. For the Brits, the 1750's through the early 18 teens were very busy from a military aspect. Some of the gunsmiths were on the Ordnance System and therefore I would think that arms for the military, and by extension their metal components, would have priority. When the Brits were not fighting the French, they were at war with Americans, Spaniards, etc, etc.
At times they were fighting all at once. I read years ago, that during the Rev War, there were more British Regiments fighting in the Carribean (spice trade) than there were in North American.