A picture of a British Guiana One-Cent Magenta which, according to previous sales, is the world's most expensive single stamp at Sotheby's auction house in London on April 28, 2021. (Photo/Agencies)

An assistant poses with the only British Guiana One-Cent Magenta which according to previous sales is the world's most expensive single stamp at Sotheby's auction house in London on April 28, 2021. (Photo/Agencies)

An assistant poses with the only British Guiana One-Cent Magenta which according to previous sales is the world's most expensive single stamp at Sotheby's auction house in London on April 28, 2021. (Photo/Agencies)