Ok. So it took me awhile to get back to this because I accidentally blew the main B+ regulator and I had to get another one.
However, I have done a bunch more troubleshooting and made some discoveries, and settled on a problem to fix (or attempt to).
Essentially, the problem is that the horizontal width is too narrow. Replacing one or both of the HOTs (on either side of the flyback) doesn't seem to help things, and the voltages are largely okay. To make this completely clear, in order to make the image fill the screen horizontally, the HORIZ SIZE pot must be almost all the way to one side.
When I change the HORIZ SIZE pot, the brightness (G2) of the screen changes dramatically. The wider the raster, the brighter the screen.
The difference is significant enough that a slight change would require adjusting the screen voltage to compensate.
In this image, you can see the voltages on the PIN MOD transistor Q505 when the image is properly filling the screen. If I back off of the size, the voltage will go down to what the Sam's say it should be, and the raster become dark. I also noticed that the difference between the Base and the Emitter of Q505 is consistently 600mV and not 1V as it says on the Sam's. For example, when I set the HORIZ SIZE so that the Emitter is 109V, the Base is 109.6V. I changed out Q505 for a new part and it's behaving exactly the same way.
