You wrote:
My breaker doesn't have any rating of amperage marked on it. There's a little light green paint on it and I carefully scraped it off where I think it should be marked, but don't see anything. It will be on the side of the CB, behind the panel.
Truthfully no I have not. Then try unhooking the load and powering up something else. Your load could also have a problem. And trip the CB1.
But what struck me as odd is the frequency gauge and load gauge not working anymore. These sets are as old as dirt. Sooner or later the A3, (Frequency Transducer) or M3, (Freq meter) will go bad. Have you measured for 120 VAC on the left side of the A3? Because if there is no 120 VAC there, the meter can not function.
The load gage will only work when you pull a load. So since you can not pull a load, why would it work?
They always did when I would start it up to make sure it was runable. I would adjust the throttle to get 60 hz. on the gauge.