Since it began as an experiment in 1896, Rural Free Delivery (RFD) Service enabled an increasing number of rural Americans to send and receive mail from their residences. To receive the service, a family’s mailbox had to be easily accessible, and on the road traveled by their carrier. Families whose homes were far away from their mailbox insured the security of their mail by attaching locks to the mailbox. Local postmasters allowed this practice, as long as carriers were provided with a key.
Manufacturers addressed this need by producing and selling specially labeled “RFD” mail locks. The official-looking locks were neither produced nor provided by the Post Office Department.
Nancy A. Pope, National Postal Museum