Looks like this RFD service began in 1902 in the United States. Here is what I found about RFD online:
Rural Free Delivery (RFD) was a service that began in the United States in the late 19th century. The purpose of the RFD was to offer free delivery of mail to farmers and individuals living in rural areas. It wasn’t officially adopted into the United States Post Office until 1902. They used locks like these to protect the mail being delivered and the road side mail boxes.