A STAMPED envelope sent to a Watford solicitor in 1841 is expected to fetch £600 in auction next month.
The envelope was sent to Messrs Cowley and Sons just one year after the Penny Post was introduced in 1840.
It value comes from the rare Penny Black stamp on the envelope.
Normal Penny Blacks are worth about £40 but this one was printed from a rare plate.
Mr Mark Stephenson, stamp expert from Spink auction house, which will held the sale, explained: 'Penny Blacks were printed from hand engraved plates.
'The first of these was called 1a, and when that wore out it was replaced by 1b and so on.
'This stamp was printed from the last plate, number 11, which was only in use for about one month and was used to produce the fewest stamps.'
The stamp will be auctioned on Wednesday, June 7.
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