Bron (Burón) argot: why those three places?

Cant, slang, cryptolects, jargon and the like are the seedy underbelly of historical linguistics, populated as they are by actors and thieves and carnies and gypsies and merchants and various other undesirables — sometimes (Polari, e.g.) all at once.

But why, I wonder, is the Bron (or Burón) argot spoken only by “itinerant coppersmiths and fabric merchants in Miranda de Avilés, Fornela, and Auvergne”?

What connects those places?  I can imagine some sort of connection betweeen Miranda de Avilés, which is on the coast of Asturias north of Oviedo in the buckle of the Bable belt (!) and the remote Leonese valley of Fornela (Forniella), 90 miles to the southwest in the Bierzo, west of the Sil River, where the isolated population spoke a distinctive local language.  Fornela, according to an article by Garcia Gil, is at the boundary of the Astur-Leonese and Portuguese-Galician language clusters.

But why (Occitan-speaking) Auvergne in south-central France, more than 600 miles away?

And why those three places, only?  And why coppersmiths and fabric merchants? (There’s also a Buron near the Picos de Europa, seemingly unrelated though.)