One of only three known surviving envelopes postmarked on the first day of the famous Pony Express service in 1860 has been
sold at auction for $US460,000.
It was among a collection of historic Pony Express mail and postage stamps offered at public auction in New York City on December 5.
The 63-item collection sold for $US4 million.
The items were owned by retired Hawaii business executive Thurston Twigg-Smith, 88, former publisher of the Honolulu Advertiser newspaper.
It was described by Scott Trepel, president of Robert A. Siegel Auction Galleries, as the most important private collection of noteworthy Pony Express material ever offered by public auction.
Although it operated for only 19 months from April 3, 1860, to October 26, 1861, the Pony Express is an iconic part of US western history.
Mail was carried by lone riders on horseback at full gallop between St Joseph, Missouri, and San Francisco, California, a distance of 1900 miles, over rocky terrain and through unpredictable rivers.
Riders changed horses at stations set every 75-100 miles.