Natchez & Bayou Sara RPO duplex handstamp
The Natchez & Bayou Sara Railway Post Office (RPO) is an example of an RPO that operated on a boat. R. F. Learned was the contractor who ran this Mississippi River steamboat route between 1884 and 1917. Boats with an 'R.P.O.' designation had a secure apartment for mail distribution. A postal clerk assigned to each trip sorted mail for post offices at landings along the river.
The Chambers shop at Lodge, Virginia, manufactured this postmarker prior to 1931. The center mortise holds four types in place: train number, month, date, and year. The set-screw at the top secures the type when it is tightened.
United States Transit Markings Catalog by Charles L. Towle and published by the Mobile Post Office Society, Affiliate 64 of the American Philatelic Society. Its catalog number is M-16-a and found on page III-N-4 of the 1979 edition.