Hello, Telf,
I doubt that both of the display envelopes have burnt-out the same digits (No.s 5, 6 & 8, going by the picture), so my suspicion would be that they aren't being driven by the display controller. That in turn may not be receiving controls for those digits from the Cpu board, via the grey ribbon cable attached to the connector marked 'Display' - though it's only used by the three designs with alpha-numeric displays, including B/o/Pinbot
According to the schematics for the similar board(s) in the contemporary game FunHouse, though, the same four lines are used to control all sixteen digit 'strobes' of each display envelope, so they must be reaching the display controller board.
Digit strobes 5 thru 8 for the upper envelopes share chip U 6 on the display controller, specifically the 'B' section of the two it contains ('A' is used for digits 1 thru 4). A similar arrangement for the lower envelope uses U 7, but with section 'A' for digits 5 to 8.
The same four lines (STX 1-4) are used to 'address' each section of these two chips, so there can't be a problem with the lines. The four sections are selected individually as required, and the problem may be with this, i.e. the selection for 5 to 8 not working. Selection is performed by half of chip U 11 - the other half gates the four data lines into the specific digit 'STX' lines. The only step between U 11 and the machines' data lines is U 16. This splits these four lines (which are common across the board) into two pairs, one pair to each half of U 11.
In short, I'd suspect U 11 and/or U 16 of the display controller board, despite the existing diagnosis