The rifle(s) most likely came from some custom gunsmith, who was also a Newton cartridge fan - or worked for a customer or three who was.
I would think the field narrow, due to the special & esoteric nature of the beast.
IMO, marking the receiver of a custom rifle, ILO the barrel, for the chambering is also unusual - as someone may want/have to change out the barrel to somthing else in it's future.
Maybe the rifle(s) are stamped/etched elsewhere, like the barrel bottom inside the stock, with a maker's mark.
You might enjoy this website on Newtons, complete with cartridge drawings, rifle & cartridge pics, old ads, etc:
http://opencarryflorida.org/256NewtonPage.html.