You missed one. Looks like the Malory has a metal box & may
not work. See this one.
https://www.ebay.com/itm/Zenith-NOS-...pDL:rk:27:pf:0
That will take 6 KV & probably a LOT more. Maybe build it into an
old HV cup with resistor/s to make it 50Meg.
BTW I worked for a big Zenith dealer starting in 1971 til the "end".
I have never seen this part. The part ## would be late 60's. Its an odd
ball. Almost all sets then used the very common 63-6444. It may be from an
early hybrid. Seemed like every one you opened had a different HV
section.
73 Zeno
LFOD !