From IPDB: A factory add-a-ball version of this game was located in France having serial number 3022. The backglass and playfield are no different than the replay version. It was factory-wired with an add-a-ball stepper unit in addition to the standard replay stepper unit . There is no option to switch between replay and add-a-ball modes; the replay unit operates only to register multiple credits when a coin is inserted. The add-a-ball unit is not a kit and is mounted in the backbox in place of the 0-9 unit (match unit), assisted by two add-a-ball relays mounted nearby. Because there is no 0-9 unit, each player’s "ones" score reel is not wired for match operation at end of game. The silkscreened match numbers 1 through 9 on the backglass are used to count the added balls, leaving the number 0 not wired. Added balls are awarded for high scores (in multiples of 1000 points only) and for Wiggler Jet Super Bonus scores. Each player can earn up to 9 balls during the ball in play, and all added balls must be played off by that player before the next player is up. The Same Player Shoot Again indicators remain lit continuously for a player from the moment the first added ball is earned until the last added ball is shot into play. We don't know how many games were wired this way. No reference to this version has been found in the Bally Parts Catalogs.

I would check the make/breaks sw. on the Game Over relay and also score motor sw. 7B.

https://www.ipdb.org/files/2777/Wiggler_Motor_Switches.pdf