I will try to find out about how many Borovlje Vierling even if I have to travel to Borovlje/Ferlach to ask the right person.
Yes, a challenging quest... my estimate would be that in either Suhl or Ferlach, possibly 1 Vierling was made for 1000 Drillinge, Bockbüchsflinten, Doppelbüchsen taken together. Or for 5000, who knows...
They were always expensive toys for big boys, and intended as a show-off-piece, much more rarely used for actual hunting. But a mail-order-enterprise such as Burgsmüller offered them too. These latter are often considered problematic.
As to the makers, several of them offered Vierlinge but few of them actually made and assembled those themselves - they just "went down the street", as a user called "Carinthia" once explained. Especially since to acquire any prowess in regulating those, necessitated much black magic and peril for your soul... Samiel wanted his dark part of the deal.
Carcano