Yes,

 I just swapped a good display & it is missing digits when swapped.

 What does this mean?

As Ajlaird said this suggests the displays look to be good, so the problem is master display or in my opinion, quite likely to be interconnector cable issues. I have a few WMS of this vinatge and most exhibit these display problems at least some of the time.

As a first step I'd look to repinning all the master display to player display and cpu to master display interconnector cables. This is pretty cheap to do and will increase reliablility. As a test with the machine on try giving the cables a bit of a wriggle and observe if this changes things.  If it does then this adds weight to the dodgy cable idea.  Also note that the way the displays are strobed is in pairs - (P1 & P3) as one pair and (P2 & P4) as the other so if the same digit is stuffed on, for example, P1 & P3 then this may suggest the cpu / display cable.