The meter adjustment jack is for the connected meter (a.k.a. counter) to either display "total coins" (meter), "total plays" (regular), or "total replays" (replays).

Credit adjust is the number of credits given per coin (1 or 2).

The "skill gage" requires a white connector to be plugged into it. I think it went to the top of the board and is hidden behind the coin box frame. Also there should be a ground wire connected to one of the mounting bolts of the shooter assembly. Yours appears to be connected.

Those taped off black wires go to the coin meter. I can see the outline of where the meter's frame was.