I've previously reached out to other owners and got some good obscure info through pinballowners primarily, but pinside lists owners too that you can PM.
AlanJs links above contain the answer to your question, see excerpt below. For home use I would prefer the AB "add-a-ball" or NOV "novelty" position. AB isn't a a true add-a-ball where balls to play are incremented on a stepper, but the Bally "shoot again" version where you get to play the current ball again once drained. This is controlled via the Extra Ball Relay which holds whilst shoot again is valid. I think Bally were cheeky promoting this as add-a-ball as a lot of machines were set or could be converted with a kit to behave in this more limited way.
There are 3 adjustments on the hi-score special features adjustment plug:
"C" Position: Hi-score feature scores credits, top center lane rollover and the center hole score a credit or an extra ball when lit.
"AB" Position: Hi-score feature scores an add-a-ball. Top center lane roll-over and the center hole scores an adda-ball, when extra ball or special is lit.
"NOV" Position: This i3 the novelty position. In this position, all hi-score adjustment plugs should be un-plugged and the novelty adjustment plug should be in #2 position. In the novelty position, no credits, extra ball or add-aball is awarded. In this position, the special awards 5000 score, when the special is lit, and the ball goes thru the top center lane or into the center hole. Extra Ball Award: The extra ball relay is energized which allows the player to shoot again without deducting a ball to play. Only one extra ball per ball in play, when an extra ball is earned, this ball must be played off before another extra ball can be awarded.