I have no connection to this but it's cool.
Ayuh, a Hoyt meter, made at the Hoyt Electrical Instrument Works located at 23 Meter Street in Penacook, NH and in business since 1904. Yes, they were a vendor of ammeters used on the assembly line for the Model T.
The item if it works is a great collector's item but alas as a magneto tester it is simply an AC voltmeter and nothing more. The letters above the voltage scale signify the usual Poor, Medium, Good, and Excellent. The problem is that there is no load presented to the magneto for the test and without an accurate tachometer to measure the frequency one cannot actually make a "GO" or "NO-GO" test of the magneto. The higher the RPM required, the greater the magneto AC voltage has to be in order for the coils to operate correctly. The instruction sheets that I have seen for these types of instruments always seem to have the disclaimer that to determine if the magneto is strong enough you must compare your readings with those of a known good working magneto but of course if you have a car with a working magneto then you could simply test your coils in that car to determine if the coils have some issue rather than the magneto. That is the important test. Hoyt and others made a lot of different testers like this but this one is neat in that it is a pocket device. Most have wooden cases and are larger.
I have one of these meters and never knew how to use it. How do you test the mag with it?
There was a discussion on forum in 2009.