Okay, okay.... this is from a page that explains how a 1947 Mills slot machine worked:

"Checks are coin-sized but have a washer-like hole in the middle that is used by a probe in the coin head to sense the presence of the check. A simple lever setting sets the linkage logic to coin or check play. When the lever is set for coin play/payout, checks that are played are captured at the coin head and sent directly to the cash box. They do not enter the payout tube. Checks permit operation of the machine, however no payout is made for a winning combination. If a vendor is attached, no vend is permitted when a check is played.

When the lever is set for check play/payout, just the opposite occurs. Coins that are played are diverted directly to the cash box. Checks are sent to the payout tube with overflow to the jackpots and cash box. The machine will award payouts of checks for winning combinations for both checks and coins played. If a vendor is attached, vends are not permitted for checks played.

Some slot players of the day would fill the center hole of the checks with lead or solder. The check would then appear to be a valid coin."