A rare Inverted Jenny is the highlight of Siegel's upcoming 2011 Rarities of the World stamp auction in New York.
The famed upside-down biplane is always a big hit with alternative investors at auction and this specimen is said to be one of the best-centred examples around.
Graded VF-XF 85 by the Philatelic Foundation in 2005, it comes with a $500,000 estimate ahead of the June 18 auction.
Siegel states that the stamp in question offers "unusually choice centering with wide and well-balanced margins on all sides, fresh and bright colours, [and a] small hinged area".
The Inverted Jenny is well known to many philatelists.
Just 100 examples of the error stamp were produced in the May 1918 US printing, all on a single sheet. The sheet was broken up by collector Edward Green later that year. Examples regularly appear at auction but in widely differing states.
This specimen is "remarkable for its pristine state of preservation... equalled by only a few stamps in the original sheet", explains Siegel. Most others have faults or disturbed gum.
Such is the interest that surrounds this historic stamp we expect it could come close to matching the Siegel auction of November 2007, when a specimen sold for $977,500.
A block of four was bought by collector Bill Gross at Siegel in 2005 for $2.7m, a world record for a philatelic auction.
Collectors wishing to make philatelic investments at the lower end of the scale can find many items of interest on the private market.
The sale will also feature a fine collection of postmasters' provisionals and a rare complete set of panes from the 1907 Jamestown Issue.