Combined Treaty Rate, Service U.S. to West Coast of South America via American and British Packets, Prepaid by Stamps. Envelope and contents from New Bedford, Mass. to San Carlos, Chile, paid by 1867, 24¢ gray lilac, F. grill + 10¢ yellow green, F. grill (damaged perfs one side), "25" and "24" credits in red (see explanation below), Panama transit cds, several colorful postmarks on reverse, Very Fine, most attractive. Scott No. 99+96  Estimate $600 - 800. The Combined Treaty worked as follows: the United States had a service which carried steamship mail between New York and Aspinwall on the east side of the Panamanian Isthmus for a flat rate of 10 cents, while the British had a packet service running from Panama, on the western side of the Isthmus, to the various South American Pacific ports for a flat rate of one shilling, or 24 cents, The combined rate of 34 cents thus paid for both legs of the service (except for Panama itself), The rate credits on this cover work thus: of the 34 cents prepaid, the U.S. kept 9 cents and credited 25 cents to Panama, which kept one cent as a transit fee and then credited 24 cent to Great Britain. $0 (Image)