It was bought by Michael Cooper, a businessman, at his local post office in Shaftesbury in Dorset. When he realised its value was missing, he exchanged it but it occurred to him that the fault could make the stamp valuable. He asked for it back and requested two similar examples. The postmistress found them for him.
He sent a photocopy of the stamp to Richard Ashton, a Sotheby's expert, who valued it. The stamp, described as a pounds 2 Indigo, bears an illustration of Edinburgh Castle.
Mr Cooper said: 'I am one of those types who always keeps a look-out, but that stamp was fairly obvious. I have always thought I might be lucky one day, but a lot of things like this must just pass through one's hands without being noticed. I expect there are some more of these stamps stuck on some old wrappers in a dustbin somewhere.'
The stamp will be auctioned at Sotheby's, London, on 10 September.