A Pony Express postmarked envelope originating from Hawaii has sold for $550,000 - a record for a Pony Express item.

It is only one of two recorded $4 Black Pony Express covers, both originating from Hawaii and carried on the same "pony trip" (or horse ride).

"The Hawaiian Pony Express cover (postmarked envelope) is one of the most important items in [US] postal history," Scott R Trepel of Siegel Auction Galleries told UPI.

Wells, Fargo & Co. Pony Express, $4, Black ($550k)

"A half dozen postmarks including a Pony Express mark trace its travel from when it began its transcontinental US journey from San Francisco after arriving from Hawaii."

Today, the Pony Express is regarded as an iconic part of Wild West history. It operated from April 1860 to October 1861.

Mail was carried by lone horse riders, 1,900 miles between St Joseph, Montana and San Francisco, California over rocky and unpredictable terrain.

The cover sold at Robert A Siegel Auction Galleries, New York.