ON April 30, Philadelphia auctioneers Freeman’s will sell flags from the U.S. Navy’s most revered ship, the Constitution.

Their whereabouts previously unknown, this group of a dozen flags spanning six decades from c.1790 to 1850 has been in private hands for more than 150 years.

The Constitution, a three-mast 44-gun frigate named by George Washington and launched in 1797, has the distinction of being America's oldest commissioned naval vessel - and the ship whose size, speed and sophistication caused a wholesale redesign of ships in the British Navy.

In 1997 'Old Ironsides' sailed under her own power for her 200th birthday before returning to her berth at Pier 1 of the former Charlestown Navy Yard in Boston.

It appears most of the colours were acquired by the collector H. Richard Dietrich Jr in 1964 via dealers Horace and Elinor Gordon. At the time they were sold by the grandson of Virgil Parris (1807-1874), former state senator from Maine and Keeper of the Stores of the Portsmouth Naval shipyard.

When in 1855 the Constitution arrived in Portsmouth for decommissioning and use as a training ship, Parris was informed that her 'light gear' - sails, rigging, spars, hull timbers, and various instruments, as well as all colours including small boat and signal flags - were to be removed and condemned as unfit for service. Parris later bought the flags at a public auction.

Highlights of the collection include four rare and extremely early US ensigns and a US Commodore's 'broad pennant' plus a royal or imperial Brazilian courtesy ensign flown when visiting a Brazilian port or returning a salute from a Brazilian warship and a British ensign of similar early to mid 19th century date. A rare French Republic commissioning pennant was possibly seized during the Quasi-War with France, 1798-1800.

The global estimate for the collection, to be sold in 12 lots, is $1m to $2m.