Originally you wouldn't have had to change anything on the board to to put it in your Tri-Zone if Firepower was using it's 6 original chips...which it must have been.
The Firepower board you have now is modified , and if you wanted to to put it's chips into your Tri-zone board then you would have to modify it for them to work.

Ah .. now I get what you mean Ken .. ok, so will fall back to the modified CPU board with a different Driver board and see what happens .
I don't understand how a fault on the driver board could cause the missings digits in score displays prob tho ..