Looking at your picture, it looks like your 'c' segment is out in displays 1,2 and MASTER. If you look at the first schematic closely where it says 'SEGMENT DRIVE FOR MASTER PLAYER 1 AND 2, the first three different-colored resistors are for keep alive, so not part of the problem. Segment 'c' is going thru resistor R3 but its hard to read because they wrote over it. If you look at the 2nd schematic, you can find segment c on the right of IC6. Follow that line straight through the right side of IC9, and it confirms that segment c is in line with R3. So replace R3 with 10K 1/2 watt.
r3 (resized).jpg
If that doesn't solve the issue, anything in that chain can be bad. IC6, IC9, the glass, or the signals from the MPU.

If you need to look up resistor color codes, just use one of the online calculators like this:
https://resistorcolorcodecalc.com/

wmsdisplayfront.pdf

wmsdisplayschematic.pdf

That is very common. Even if not the problem, it takes 5 minutes to reflow them and you don't have to worry about them again.

If you want to pinpoint the exact connector and pin driving the knocker, you can grab the schematics from here and see if you can find it.
https://www.ipdb.org/files/856/Williams_1980_Firepower_Schematics_paginated_from_manual_dated_March_1980.pdf

Does the plunger move freely with your finger?

My advice is to first find where the knocker coil is connected in the schematic and concentrate on that. Otherwise you're just chasing your tail with other problems. If pushing on that 40 pin connector causes issues, it may be something you need to fix eventually -- but it can wait until you fix the knocker issue first, right? It is just my opinion, but I would work one issue at a time.

Page 9 of manual, and page 24 of schematics....

knock1 (resized).jpg

knock2 (resized).jpg