I've attached a photo of the booster circuit I added. It's on the board with the orange drops.
Close, but no cigar.
Yet.
I tried mounting the 12LP4. I raised the scanning bridge to the next higher set of holes which centers the yoke on a 12" CRT. However the 12LP4 is 3/4" longer than a 12VP4. The yoke has a large adjustment range to accommodate both the 10MP4 and the 12VP4, but not enough to handle the 12LP4. I was about to give up and consider staying with the 10MP4 and then having to decide whether to mount it in the 1-128 cabinet or the 1-076 cabinet. 10" would make most sense in the 1-076, but the 1-128 cabinet is the one our family kept.
But, not all is lost. It turns out that the chassis has another set of holes for a 12" CRT located 3/4" farther back that would make the 12LP4 fit perfectly. This must be the Swiss Army Knife of chassis. It was used for a large range of sets including one that used a 16LP4 CRT (at least the bridge was). The 16LP4 version has a boost circuit for the screen that uses a 6X4. There is a hole already punched for it in the bridge. I could have gone that route, but I would have had to reroute too many wires. It would have been a lot of work and the work would have been obvious. So, I used a silicon diode version of the circuit that I could hide underneath.
But, the bell of the 16LP4 is 4 inches longer than the 12LP4. I'm guessing but it seems that they had designed the chassis to be able to use the 12LP4 but never built them with it. The 12VP4 was probably less expensive, especially since they could leave out the 6X4.
When I do move the bridge back, the cross brace may not have a set of holes for it. It had one for raising the bridge, but I suspect there won't be a matching location for both raising and moving the bridge back. I may have to make a different cross brace. Not a big deal since it's just a flat strap with a bend and each end.
John