I posted this one about 1 1/2 months ago. This is from the Livingston, TX casino before they closed it.

The card was actually a machine key. The slot machine was inoperarable when removed and the attendant lights would start flashing. (Tough to steal errrr GET -- yea that's it.) They never had slot cards for their slots only an ID card to enter the "entertainment center" -- sort of private club thing.

I had assumed that it was like the car anti-thief device that just shorted certain circuit paths to allow operation.

Someone mentioned that it was a card that held credits that you paid in advance or at the cashier. I think some states tried to monitor the total amount allowed lost per person per day. That was not the case here.