This service began in 1901 under the "Railway Mail Service" and the postmarks used the initials "R.P.O" (Railway Post Office). This service came to
the "Sea Post" 01. October 1912 until it was interrupted early in 1917, due to the German declaration of unrestricted submarine warfare against all ships.
The service was resumed in 1921, again under the "Railway Mail Service", until it came under the control of the "Sea Post" from 1925.
It continued until February 1942, shortly after the Japanese declared war.
Service 1912 - 1917 - Postmark R3
- Nr. 1 - S.S. Brasos (see cover No. 2 - on the reverse side is the name of the ship)
- Nr. 2 - S.S. Caracas (1915-1916)
- Nr. 3 -
- Nr. 4 -
- Nr. 5 - S.S. Maracaibo (1915-1916)
- Nr. 6 - S.S. Philadelphia (1915-1916)
- Nr. 7 -
- Nr. 8 -
Service 1921 - 1923 - Postmark PR4
- R.P.O. Nr. 1 - S.S. Ponce?
- R.P.O. Nr. 2
Flag Cancellations (1928 - 1935) - Postmark PR7
- SEA POST Nr. 1 - S.S. Coamo 1928-1934
- SEA POST Nr. 2 - S.S. San-Lorenzo 1928-1931
- SEA POST Nr. 3 - S.S. Borinquen 1931-1935 (siehe Beleg 4: S.S. Borinquen mit der Nr. 2)
Cockrill, Booklett No 54 und No 55
Hosking, Seaposts of the USA
Postage paid commercial postcard with cancellation PR1 to Cockrill. Postmarked on 12. February 19XX. Addressed to New York State.
Postage paid commercial cover postmarked PR3 to Cockrill with the number 1 in the killer. Postmarked aboard the ship "Brasos" on 07. October 1914. Addressed to New York City, N.Y.
Postage paid collector's cover postmarked PR6 to Cockrill with the number 1 in the killer. Postmarked aboard the ship "Coamo" on 26 November 1931. Addressed to Babylon, N.Y.
Postage due collector's cover with postmark PR7 to Cockrill with number 2. Actually number 3 to Cockrill belongs to this ship. Postmarked on board the ship "Borinquen" on 23 April 1931. Addressed to Santo Domingo City, Cuba.
Postage paid collector's cover postmarked PR7 to Cockrill with the number 2. Postmarked aboard the ship "San Lorenzo" on 04. December 1930. Addressed to Easton, Penna.
Postage paid collector's cover with postmark PR11 to Cockrill. Postmarked aboard the ship "America" on her maiden voyage on 14. August 1940. Addressed to Westpotz, Connecticut.