[Tom Albrecht]

Not sure why they left the voltage off -- sounds like an oversight.

A good way to figure out what the voltage rating needs to be is to operate the set and monitor the DC voltage across the cap, both during warm up and while it is running. Then add a nice safety margin of something like 20%, and choose a cap with that voltage rating or higher. If you have reason to believe there is a large amplitude AC signal across the cap (not merely coupled through it), you'll need to increase the margin.

Micas are generally available in 300, 500, and 1000 V versions. In most circuits, 500 V is fine, but there can be a few key exceptions -- in particular if a low value mica cap is used as a feedback path from the flyback transformer to the horizontal AFC. That one needs a 1500 or 2000V rating, which is not so easy to find. Usually I don't replace that one unless I have a clear reason to do so. It is not used in all sets, but many do have a small value high voltage mica in that particular circuit.

Unfortunately these days I see lots of failures of mica caps, so the general idea that most of them can be left alone is becoming less and less true as more time passes. When I'm restoring a set for someone else and want maximum reliability, I replace all the micas in the sweep circuits, and also any in the IF or video circuits that have any significant DC voltage across them -- those seem to die faster than those in tuned circuits with no DC across them.