1940 Jennings Modern Vendor - EM Slot

[Rick]

I'm working on a 1940 Jennings Modern Vendor slot I picked up a while back.
This is my first electro-mechanical machine to work on and I have a question for you guys.
This is a "play for credits" machine that Jennings made for a very short time in 1940 for states that didn't allow gambling.
There is a counter on top of the machine that keeps track of how many credits you've earned.
On the front of the counter is a counting arm and a dial.
I know the dial shows how many credits I've earned.
What's the relationship between the dial and the arm?
I can't figure out how the machine knows how many credits to give me.
I should "pay out" between 1 and 5 credits for each win.
I'm guessing this style of counter is somewhat common in other uses.
Anyone know how it works?