The 1855 Swedish 3-skilling banco yellow is the world’s most valuable stamp.
It sold for over US$ 1.3 million in 1990.
Why so much?
It's very valuable because it is a mistake. The stamp was printed yellow in error, when it should be green. Only one copy is recorded.
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How about looking at some of the rarest stamps on show in the UK?
Philatelic Rarities Website: www.collectbritain.co.uk/collections/philatelic/
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