Postal workers were assigned keys as needed. Carriers and employees who gathered mail from collection mailboxes were assigned keys for the boxes on their routes. Postmasters were held responsible for a post office’s keys, including the registered mail lock keys. Each postmaster received detailed instructions on how to handle the locks and keys provided for his or her post office. Surplus or broken locks and keys were never to be discarded, but returned to Washington, D.C.’s headquarters or Mail Equipment Shop.
Nancy A. Pope, National Postal Museum