A cover bearing a Southern Letter Unpaid handstamp is estimated at $15,000-20,000 ahead of a sale of US postal history at Robert A Siegel in New York on November 20-21.
Siegel describes the lot as: "one of two recorded 'southern letter unpaid' covers from Charleston, South Carolina, the place where secession started and the first shots of the war were fired on Fort Sumter."
The handstamp was first used in 1861, following the outbreak of the civil war.
The hostilities meant that it was no longer permitted to send mail from North to South, although it was possible to send it from South to North via the Louisville, Kentucky office.
Southern stamps, however, were no longer valid in the North and the postmaster at Louisville was ordered to remove them - a huge task as the mail was piling up quickly.
He got around the problem by stamping "SOUTHN. LETTER UNPAID" on each envelope. Today they are a highly desirable piece of philatelic history. Only 29 covers are known to have survived.
Earlier this year another Southern Letter Unpaid cover sold for $52,000 at Siegel, an increase of 31.2% on a $40,000 estimate.
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