A barrel thread that becomes a varnish thread. Interesting!
There is no single, definable "violin finish" or "violin varnish." In the past five centuries there has been a huge variety of finishes applied to stringed musical instruments. Some are soft, some are hard, some are fragile, some are not so fragile. And probably every natural resin under the sun has been utilized on an individual or regional scale.
There is a definite, documentable history of highly colored varnishes being used in the areas of old Northampton Co. (now Northampton and Lehigh) and eastern Berks Co. as well as into upper Bucks Co. Many of these are resin varnishes in an alcohol base, what most people refer to as 'violin varnishes.' They are not the greatest thing in the world to be applying to a gunstock intended for exterior use, but conversely, by the time these finishes seem to have become popular in the region, I don't think these guns were being used too hard and I doubt anyone was dragging their fancy Kuntz rifle through the rain and mud for weeks at a time.
Anyway. Some "violin finishes" are spirit based, some are actually oil based. Some involve no heat at all in manufacture, some involve scary heat around flammable materials initiating the rumination while making them that you had a full asbestos suit.
Nowadays, a whole lot of people call just about any varnish or wood finish with resins added a "violin finish."