Like Daniel said, more than likely a connector problem, but displays that are missing digit segments can be because of failing/ failed resistors located on the master display board.
Simply have a look and any obviously burnt resistors will need to be changed, but its a good idea also to check each one with a multimeter to see if they are open circuit or the wrong value.

If you can get a manual with circuit diagrams they will show you the correct values for these resistors, in fact any Williams machine from that era will use the same displays too.

Its also a good idea to give the edge connector on the actual display board a light sand with wet "n dry paper and clean the gunk off with metho. I did this on a Flash I fixed up and it did improve it a bit.