Looking good, Rex, but be sure to leave enough work for Duncan to figure out himself.
If the resistor's location would permit it to be near cathode potential, then it might be a bias resistor.
In Coolidge tubes with three terminals at filament end, the isolated one doesn't go to a hot cathode (in my experience).
It goes to a metal part behind and around the filament, that goes by names like focusing cup, grid, or Wehnelt electrode.
That part can be optimized for zero bias (for simplicity), or for bias 100-200 volts more negative than filament-cathode.
In the second case, bias can be developed with a resistor, giving some HV current regulation as well as sharper focus.
My own mains-frequency XRT has that resistor and filament winding under the wrapping of one secondary winding. Took a while to figure it out from external resistance measurements.
Commonly, VF and VH are both 60 Hz AC, for "self-rectifying" Coolidge tube operation as Rex speculated. There's no HV current in half of each mains cycle. The average HV current is DC through the secondary windings, amounting to more than a few ampere-turns and shifting core flux toward saturation on one side. Sometimes (I don't know enough to say usually) compensated with a diode and power resistor in parallel, in series with the primary winding (and could be outside of HV tank).
Also, as a reminder, nominal voltage
of X-ray transformer secondaries
is generally peak, not RMS. The nameplate and meter on radiography systems say kVp.
Oh, and my old reverse-engineering post is still up, with image order reversed: viewtopic.php?f=11&t=4805&p=27643