See here from earlier today:

https://pinside.com/pinball/forum/topic/bally-ek-connector-issue

You don't need to bare the wire. Provided you didn't insert the wire with something too thick that may have spread the terminal in the connector (so it doesn't pierce the wire insulation), it should be ok but you better check continuity:

Grab your multimeter. Set it to low ohms range. Put one meter lead on the left side of resistor R1 shown in your picture above that's next to J1 pin 1. Insert the other meter lead in pin 4 at the displays connector. Do you get zero ohms reading (i.e. not open circuit).

For your sanity do the same with the 10's digit signal so you see how the meter should respond which is the right side of resistor R2 on the MPU board and pin 5 at the displays connectors.

On the 200 ohm resistance range (this is the correct setting to test continuity), your meter is telling you that on the 10's digit enable signal, there's 0.7 ohms resistance which is good continuity from the MPU board to the displays.
Touch both your meter leads together in resistance mode, the reading you get is a closed (short) circuit. That reading or very close to is what you're looking for when testing continuity.

If the meter isn't budging from "1", that tells you there's an open circuit on that black wire between the left leg of R1 and the displays at pin 4. You need to find out where the break in connection is.

Where you've reinserted the black wire into the J1 pin 1 terminal, put your meter lead in the cut end of the black pin 1 wire where you should see the internal copper core of the wire. Put the other meter lead on the left leg of R1. If the meter still doesn't budge and assuming you've made contact with the black wire core, then the re-insertion of the wire into the connector hasn't made contact with the terminal in the connector.
If you do measure around 0.7 ohms, then you need to follow that black wire downstream to the displays to find where it's lost connectivity.