Cornelius Vanderbilt founded the Atlantic and Pacific Steamship Company to compete with the Pacific Mail Steamship company and the United States Mail Steamship Co. on the Panama route between San Francisco and New York. The Atlantic and Pacific operated Aspinwall to New York portion of the route from 1860 through November 1865.
Gyayaquil, Ecuador, September 29, 1860 backstamp, PAID-TO-PANAMA marking in red, per Pacific Steam Navigation Company to Panama, and Atlantic and Pacific Steamship Company from Aspinwall to New York, New York Steamship "STEAMSHIP 10" 10¢ rate for incoming steamship letter, for distance under 2,500 miles.
Sonsonate, El Salvador, October 10, 1862 dateline, Acajutla October 11, 1862 circular date stamp, per Pacific Mail Steamship to Panama, then overland across the isthmus, Atlantic and Pacific Steamship Co. to New York arriving November 3, 1862, rated 20¢ due as an incoming steamship letter for distance greater than 2,500 miles.
Guayaquil, Ecuador, December 31, 1862, endorsed “p. vapor via Panama” New York, Atlantic and Pacific Steamship Co. from Aspinwall to New York, "STEAMSHIP 10" 10¢ rate for incoming steamship letter, for distance under 2,500 miles.
Guatemala City, Guatemala, July 7, 1864, 2 reales internal postage, per steamship via Panama, Atlantic and Pacific Steamship Co. from Aspinwall to New York, arrived New York August 6, 1864, New York "STEAMSHIP 10" marking, rated 10¢ due as incoming steamship letter regardless of distance.
Valparaiso, Chile, October 18, 1865, 1/- rate marking, indistinct PAID-TO-PANAMA in red, per Pacific Steam Navigation Co. British packet to Panama, and Atlantic and Pacific Steamship Co. to New York. Panama November 4, 1865 transit marking, New York "STEAMSHIP 10" rated 10¢ due as incoming steamship letter, regardless of distance.