I've searched all around but have not come across information about this exact problem (or perhaps I did not understand the explanation if I did). I've got a Pink Panther that will not display the number 1 in any digit position on the displays for Player 3 and Player 4. That is, all six digits will display any numeral except "1". If the score is such that a digit should display a "1" that digit is instead blank. For instance, if your score is 210010 you would see 2_00_0, if it were 111120 you would see 20, and so on.
The problem is not with the displays, they work normally in the Player 1 or Player 2 positions, and displays that worked normally in those positions exhibit the problem when installed in the Player 3 and Player 4 position. I've tested the game with either the Player 3 or Player 4 displays unplugged (as I'd read that one bad display can cause problems to the other in the group), but that makes no change. For good measure I installed the MPU board from Haunted House in the game and ran the Display Test in Diagnostics and it was able to show every numeral in every position.
Looking at the MPU board it is clear that someone has tried to tackle this (or a related) issue previously. The 7404s at Z17 and Z24 had been socketed and replaced by a previous owner with 74F04s. I reseated these chips. I reflowed solder to these sockets and checked continuity between all pins and their destinations. I installed a socket at Z26 and while I did not have any new 7404s on hand I swapped locations of the previously replaced chips at Z17 and Z24 with the very carefully extracted original 7404 from Z26. No configuration made any difference, so I think I can count all three chips as good.
The previous owner also socketed and replaced Z16 and Z19. From what I have read, you would replace chips in this area if you had stuck on segments, which I do not have. I reseated these chips and reflowed their sockets just for good measure.
I have checked continuity of every trace between the the 7404s at Z17, Z24 and Z26 with the edge connector and the 74154 at Z33. I have checked continuity of every connection listed in the schematic between Z33 and the 6532 RIOT at U5.
If it's not become clear yet I am no electrical engineer but I am a pretty good electronics mechanic, I may not understand the circuit logic but I'm real good at installing sockets. I've noted that the working (but really hacked up) Haunted House board has had both the U5 RIOT and the Z25 74154 replaced, which were my next thoughts as to what to replace next as they're the next chips in line on the schematic.