Pacific Mail Steamship Company

The Pacific Mail Steamship Company operated the San Francisco to Panama portion of the Panama route from 1848, and took over the Aspinwall to New York portion of the Panama route beginning in November 1865. 

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. packet to New York arriving November 3, 1862, rated 20¢ due as an incoming steamship letter for distance greater than 2,500 miles.

Valparaiso, Chile, June 17, 1868, Valparaiso Crown Circle, paid 1 shilling postage per Pacific Steam Navigation Co. British packet to Panama, July 4, 1868 transit, then per Pacific Mail Steamship Co. to New York. July 12, 1868, N.Y. Steamship, rated 10¢ due as incoming steamship letter, regardless of distance.