A bizarre one, but it illustrates the benefit of having two similar machines! I've never seen a schemeatic for these displays, does anybody have one available? The board layout drawing is available at wwww.firepowerpinball.com I guess it's possible that something in a cable in one of the Gorgar displays is imposing a fault back to the master display.
I'd follow Ninos' procedure but also at the master display end on the P1 cable look closely at the green wire on pin 13 and on P2 the green wire is on pin 7 - the green is for the e segment.
Loooking at your picture the board has been worked on before - look at the resoldered pins on IC9 near top left and the replaced resitors on the P3, P4 segment drives. As Nino says they are 10K 1/2W and at least one of yours needs replacing - I'd let my hair down and do the lot.
