Thurston Twigg-Smith, former publisher of The Honolulu Advertiser, is to auction what is believed to be one of three known surviving historic envelopes postmarked on the first day of the Pony Express.

The envelope is among 63 items that are owned by 88-year-old Twigg-Smith.

Robert A. Siegel Auction Galleries in New York will conduct the auction. The auction house estimates the collection's value is $2.5 million or more.

The envelope was postmarked April 3, 1860, and is valued at $300,000.

Twigg-Smith also will auction one of two surviving Pony Express letters that originated in Hawai'i. It is valued at $500,000.

The Pony Express lasted 19 months, from April 1860 to October 1861, reducing the elapsed time for coast-to-coast mail delivery to only 10 days. The service ended two days after the Transcontinental Telegraph connected Omaha, Neb., with Sacramento, Calif.