You didn't say which game this is, but that looks to me like a lightbox advance unit, not a ball count unit. On Italian GTB games, and later year (around 1976) domestic GTB AAB games, the WOW function was on a separate stepper unit from the ball count unit.
On these games, the ball count unit starts at 5 and counts down toward zero (game over) as each ball is played. During the game, the ball count unit never goes up. Instead, when a WOW (add-a-ball) is scored, the lightbox advance unit lights from one to five WOW indicators on the backglass. Each WOW indicates one add-a-ball. So, for each ball in play, you can win up to five add-a-balls, but no more.
It is the lightbox advance unit that steps up and down as WOWs are made, and then played off. When the ball drains, if one or more WOWs are lit, then the lightbox advance unit steps down, and the ball count unit doesn't move. In other words, the ball count unit will only step down when the ball drains and there are no WOWs on the lightbox advance unit.
The reason that the lightbox advance unit is so complicated is because the later GTB AAB games could be adjusted to be played in NOVELTY mode rather than AAB mode. In NOVELTY mode, it is not possible to win any add-a-balls. This setting was provided for those locations where even winning an extra ball was considered a gambling winning. When in NOVELTY mode, a WOW scores some very large amount of points. That means that the player may score several hundred thousand points during the game. Since the score units only go to 99,990, there needs to be some way to keep track of the 100,000 score level when the game is in NOVELTY mode.
So - when the game is in NOVELTY mode, the lightbox advance unit serves a completely different function. Instead of counting add-a-balls, it keeps track of 100,000 score levels. It works just like an old bulb-scoring woodrail in this regard. Each time there is a carry from the 10K score unit, the lightbox advance unit steps up one step. At each step, it lights the next higher 100,000 point score on the backglass - 100,000; 200,000; 300,000; and so on.
The rather complicated circuitry on the lightbox advance unit is there to provide functionality for either AAB or NOVELTY mode. The adjustment plugs in the game that switch between the two modes cause a fairly significant wiring change in the game. So it's somewhat non-trivial to fully understand the circuitry of this unit.
- TimMe