from another source...
"A CRT with weak emission can produce a picture that's dim overall, and if you crank up the contrast and brightness in an attempt to improve it, the picture can get a silvery or almost-photo-negative appearance. Kind of hard to describe, but easy to recognize if you have seen it before."
This very much describes what was seen on yours, where it's hard to tell if you have good HV just by looking at yours, there does not appear to be any blooming, a good sign of it being low, but the only real way to be sure is to use a HV probe.
ETF has a sub guide.
https://www.earlytelevision.org/pdf/...guide_1959.pdf
any in the group 7 if I remember, can be used.