Dave, thanks for all the references. My eyes are going buggy after looking through various charts and sources tonight. This chart seems pretty good:
http://www.dave-cushman.net/elect/wiregauge.htmlI'm thinking the closest match seems to be Washburn & Moen gage -- especially at the low number end of the scale. The highest number on the plate, 14, doesn't seem to fit any of them, since the minor diameter I measured is larger. However I could easiy be off in measurement, as all I have is a caliper, and the jaws don't even fit in the hole. Even then, the W&M gauge is closer than some of the others.
I looked up Washburn and Moen:
https://en.wikipedia.org/wiki/Washburn_and_Moen_North_Works_Districtand they were located only 85 miles away from here in Worcester, MA, and the vendors at the Bernardston, MA show might even have come from Worcester. Coincidentally, my father-in-law graduated from Worcester Polytech, which was founded by Ichabod Washburn.
So it seems at least a good possibility that this was a Washburn and Moen Screwplate, possibly even originating in Worcester, MA where the wire company was located.
pitch:
I do still wonder about the two weird fractional pitches -- why in the world were they chosen?
re. usefulness:
It might be fun to make a few taps to match, by running some turned drill rod through the screwplate in some useful small sizes, and then fluting and hardening -- as I did for the acme tap I made.