<-- Mobile Post Office Society
"MPOS has published the definitive studies of postmarks from the Railroad, Station, Route Agent, and Messenger services, and the Railway, Highway, Streetcar, Terminal, and Air Mail Field Post Offices as well as the Transfer Offices and Clerks in United States philately. Most recently, MPOS has added a focus on the USA's Rural Free Delivery (RFD) service and postmarks that were used around the beginning of the 20th century. 1996 saw the start of a study of "TRANSIT" markings used at intermediate Post Offices.
The research and work of many years culminated in the completion of the "United States RPO Postmark Catalogue", begun by the late Charles Towle, and completed through the efforts of MPOS Past President Fred MacDonald, along with the work of many dedicated philatelists."