(NOTE: I had posted about this about a month or so ago, but the recommendation was to fix something that I have verified is not the problem.)
The MPU is an AllTek, and the memory had been cleared.
All the displays except for #3 work fine. The leftmost digit for the display @ #3 is always out no matter what display units are swapped around, so it is not the display units. There is a daisy-chain wire that goes to #3 from #4, but I have verified continuity (while the wire harnesses are connected) between the TP2 between each display unit, so that wire is not the problem.
I have also tested continuity (while the wire harnesses are connected) for the single unique connection between display #3 & the MPU (the strobe line?) from the point on the display to the resistor on the MPU, so that wire is not the problem either.
I can't see how this is not some problem with the MPU board. What other stuff should I be checking?