A retrofit, automatic audit system for a door-access, coin-operated newspaper rack machine is disclosed. The system operates in conjunction with the standard door-lock mechanism and includes a retrofit purchase-mechanism mounted on the machine. The purchase-mechanism includes an electromechanical audit...http://www.google.com.au/patents/US4845484?utm_source=gb-gplus-sharePatent US4845484 - Retrofit, newspaper tracking audit system for newspaper rack machines
