Quoted from Roblafr:HowardR, does relay P also increase the ball count? I haven't tested anything yet. Just curious as this relay is called "add player unit relay"?
There's some good background info on the subject of the player unit and coin unit, skip down to "Player Unit Theory of Operation":
https://www.planetimming.com/Pinball/troubleshooting/Player%20Unit%20Troubleshooting.pdf