A cover displaying Martha Washington's free frank has sold for $32,500 at Robert A Siegel.
The lot was among the highlights of the George J Kramer collection of US domestic mails (1776-1869).
It was sent from the Washingtons' home at Mount Vernon on November 6, 1801.
Martha was granted free use of the US postal service following her husband's death in 1800. She died just two years later, in 1802, meaning that examples of her free frank are highly rare.
The present specimen is one of only four surviving examples and was sent to her great-grandaughter's husband, Robert Lewis, concerning a financial issue.
The Filstrup cover, featuring a rare use of the 1860 90c, also sold well - achieving an identical sum of $32,500.
The stamp is the highest value in the 1851-1857 series. Very few were issued, with the majority recalled shortly after the demonetisation of stamps in the weeks after the outbreak of the civil war.
Today there are only six surviving covers to display the stamp. The present lot is the only known example sent domestically.
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