A first day cover featuring six examples of the 1898 Trans-Mississippi issue stamp from the 1c to the 10c is expected to make $50,000-75,000 at Robert A Siegel's November 6 sale in New York.
The cover is described by the auction house as "a phenomenal rarity, and one of the most important classic commemorative first day covers extant".
It is the only example known to exist.
It features Washington DC purple registry ovals and bears a typewritten address to Herman Appel, New York.
The Trans-Mississippi issue was released in 1898 to commemorate an exposition in Omaha, Nebraska.
The initial design for the stamps, coloured frames and black centres, would have necessitated a two-stage printing process - however the outbreak of the Spanish-American war meant that presses were being used to print revenue stamps.
This ensured that all of the stamps were printed in single colours.
They feature identical frames, although each interior is different - showing a variety of different scenes from the old west.
A 2c Trans-Mississippi pre-first day cover is also offered, with an estimate of $7,500-10,000. It dates to June 16, 1898, the day before the stamps were released, and is one of just two examples extant.
A fine example of a $1 Trans-Mississippi stamp sold for $60,000 in New York in November - up 1,614% on its $3,500 estimate.
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