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The Magnavox T931 chassis I have uses the 6BK4B -
don't know how long-lasting they are, but would be happy to get a spare or two.
I remember the Xray scare when I was at Motorola - don't recall which tube it was (I worked on B&W design), but I do recall that the xrays were internally shielded by the electrodes if they were straight - but some tubes were manufactured with the electrodes crooked, and xrays could escape from the tube (at the bottom, I think). In the Magnavox t931 chassis, the tube is installed with the bottom toward the back of the set, and that side of the shield box is open - the 5 other sides are solid. (1967 model, 25AFP22 rectangular CRT, would be called 23V today).
HV regulation was abandoned in later color sets when it was discovered that the sweep current could be made to track the high voltage variation with beam current- so on bright pictures, the HV drooped, which would have increased the size, but the sweep B+ also drooped and the size remained constant. Part of the internal factory design specs was the maximum percentage of size variation with brightness that was allowed. Regulated sets had no HV droop, but then the sweep current had to be maintained constant under varying beam current loads - probably easier in sets where the video b+ comes off the line instead of the flyback like later transistor sets.
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