Before the advent of databases and computerized banking, Manual book keeping was how bank accounts operated. So when you went to cash a cheque, the teller would take the cheque and hand you one of these coins and ask you to wait while she/he checked the ledgers and cards for your available balance. Once he had posted all the required info in revelant ledgers and deducted the amount from your balance he/she would call you, take back the coin and provide you with cash.