I disconnected one lead of the cap for testing.
It's a little hard to see in the glare, but the eye tube's shadow is fully open, showing no leakage under 450 volts, using a new 1500-pf mica as a comparator.
I guess I will call this cap C290 after the CTC-2B schematic. Whatever it's made of, I'm calling it good enough for this application.
Now the trick will be to install it without torching everything else in the neighborhood, including parts I previously replaced.
Maybe I will install all new components and leads on the transformer first, slip the pre-wired assembly through the hole, and then connect up.
Phil Nelson