It's an additional playmeter that counts paid credits. Subtracting the readings from the two meters will tell the operator how many free games have been awarded by the machine. If the games is either too generous or too tight, he would possibly make an adjustment in order to compensate for either situation.

I haven't seen too many games with the two meters installed. I would guess the red holes would be where it would be mounted.

I’ve only seen cash play meters in Gottlieb’s sample games. My Joker Poker is a sample and has a pair of meters in it.

Gottlieb AAB games have factory wiring for an optional meter, too. When I restored my Lucky Hand in 2014, I grabbed a meter from a parts machine, zeroed it out, and installed it. As of today it reads 1,415 games. That would have netted me around 350 bucks at 25¢ a play. At this rate, it’ll take another nine years before it pays for itself. I better start charging money. It’s my wife’s favorite game.