Increasing capacitance across the damper made a difference. That cap (C72) is listed at 120pf/3KV, and I don't keep many high-voltage caps around, but I found a couple of 82pf/5KV ceramics and connected one across C72. That bumped out the width to nearly fill the mask. Wiring a second one in parallel increased the width to overscan the mask slightly.
The tradeoff is that with lower HV, the screen is less bright. With two 82-pf caps in parallel, the HV sinks to only 7.5KV and the screen is noticeably dimmer than before. Maybe I'll go with one cap and call it done. I'd rather have a bright screen with slivers of black on the edges than a dim one. There is still slight non-linearity, but not enough to be annoying when watching a program.
For the record, I also tried adding a .1 cap across L21. It improved the linearity very slightly, but at the expense of width.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html