Here is a picture of mine, was working with a legit board properly.
When you say USB are you trying to hook it up to a computer, pi or something similar?
The coin mech goes into the daughter that then goes to the Jamma / credit. Therefore, I would guess that two of the ribbons are what you ultimately need making a switch, then you could run that into your keyboard emulator as a button. It might need power also though, which you would have to work out through looking at the specs. My guess is that some of the wiring went to the credit part of the multi boards so you could switch games and keep credits. Hopefully that helps a bit.