Was the game made for the USA or another country (like Japan ??)

Honestly, it may be factory installed then.
I remember reading somewhere that the first game exported to Japan was Data Easts Tommy. As that had 3 motors, it got electronics certification for pinball machine containing 3 motors.
For games exported after that, it was cheaper to add extra motors than get a new certification.

Here's a post from 2002 explaining the motors for Japan:
https://groups.google.com/g/rec.games.pinball/c/IZLLYlpR3Ts/m/DRuoWrELyWIJ

(and after reading this - does your game has a shaker motor too ?

As your game has a label with Japanese markings on it and original Data East motors, that seems enough proof ?
Check the label with serial number too if it has an indication of country (inside cabinet, right side, or outer backside of cabinet).

Just look at it, all the wiring looks original too.
If an operator were to add this as a hack, they would use cheaper motors (cpu fans that cost $10; not a complete Tommy motor that costs like $100), and would just hack it together with speaker wire or other electrical wiring laying around and solder it on somewhere, he would definitely not do all the trouble of finding and continuing the original colored wiring and use the correct type of molex connectors. (Unless you're someone like HEP who has all the correct colored wiring laying around, but there are not many people in the world who have.)

I think you can be 99 percent sure the machine left the factory like this with additional motors installed.