The picture below shows how we understand the credit unit to work.
Green arrow: credit increment solenoid, which actuates...
Purple Arrow: credit unit EOS
Red Arrow: credit decrement solenoid,
Blue Arrow: credit zero position switch

From pinrepair, first two steps in the sequence:

"Williams Start-Up Sequence.

1.When the start button is pressed, the credit unit is decremented. This is done through the credit unit zero position switch and the 2nd coin relay (and the ball count zero position switch).
2.The end of stroke (EOS) switch on the credit unit decrement coil will energized the coin relay."

When we add a credit on the front door, the credit unit is decremented. But...when a game is started, the credit unit is incremented.
So the credit unit appears to work opposite to what we'd expect.

1. Could this be as simple as having a connector in back-to-front...or something more sinister?
2. This doesn't allow the start-up sequence to occur, as the credit unit is incremented rather than decremented and therefore the EOS on the credit unit does not close (step 2 of sequence above).