The "Citizens Key" was just that, a numbered key, which maybe a few of the area business owners carried. If the police were needed, you would put your key in the slot and turn, which would send a signal to the station house. The key could not be removed until an officer opened the front door and released it from the inside. This cut down on the prank calls I'm sure.
If an officer used the box, he would open the door and there usually was a switch that could be placed either in the "On Duty" position, or the "Wagon" position. Some departments would have the foot officers check in at each box on a regular timed basis to determine their location. If the officer had arrested someone and needed transportation, he would call for the "wagon" to come and pick up the detainee.
