Thanks for the advise Nick!
My file is a tad large, but if I can get the slug to rotate freely enough I ought to be able to get a screw driver to work with it after filing. I can't comment on the 21-CT-55 layout, but it would be kinda awkward to adjust the linearity control on the CTC-4 with pliers considering they wedged it between the damper and the HV cage. Not what I'd call a particularly good good place for it, but at least it is not as buried as the same adjustment in the CTC-16 (I tend to wear rubber gloves when doing live chassis horizontal adjustments on my 16's as the controls tend to be cramped, and uncomfortably close to dangerous voltages and searingly hot output tubes
).
I'll try some other HV rect. tubes, but doubt they will fare better than what is in there. The tube in there is perhaps the first 3v filament HV rectifier that I've seen that will peg the needle on my tube tester, although I've heard the mantra that the best tester of TV sweep tubes is a TV set.