The way these displays work, a strobe signal has to be present for the digit to light up. The way you have missing digits, and some distorted digits, I think you have strobe issues. Below is the strobe table out of the FP schematics. It is pretty much the same for all system 6 games.
Screen Shot 2022-06-08 at 12.41.18 PM (resized).png

100k is stobes 1 and 9, and the missing 1k are strobe 11. The master display strobes you are missing are 15 and 7.

I would check to make sure these signals are being generated at the MPU board before I looked at working on the master display board. They all come out of IC6 on the MPU (74154). If you have a logic probe or oscilloscope, you should be able to detect these strobe signals. I would then trace the wiring down to the master display board. Honestly, if you determine the problem is on the master display board, you may be better off replacing your displays with LED units, as there are a lot of obsolete driver chips on that board that can't be purchased anymore.

I think you have it licked. I forget if the strobes activate on the high signal or the low signal, but a stuck signal either way means it is not working. A high+low reading would indicate a the strobe is working.