I've seen several entries in different cities of the Missouri area that use the abbreviation of
R F D sometimes followed by a numeral for the address of a person.
R F D stand for?
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RFD = Rural Free Delivery, referring to those people on the rural mail routes. The numeral will refer to the route.
The abbreviation is noted in the introductory information of a 1914 Missouri directory.
Wikipedia also provides a good overview of Rural Free Delivery.
Rural Free Delivery (RFD) is a service which began in the United States in the late 19th century, to deliver mail directly to rural farm families. Prior to RFD, individuals living in more remote homesteads had to pick up mail themselves at sometimes distant post offices or pay private carriers for delivery.