London, May 24 (IANS): The only known misprint of an 1855 three shilling stamp retained its title of being the world's most expensive stamp when it was auctioned in Geneva.

The stamp, called the Swedish 'Treskilling Yellow', was bought by a group of buyers who wanted their identities and the winning bid be kept confidential, said auctioneer David Feldman.

Feldman did not reveal whether the sale matched the record it previously set in 1996 of 1.59 million pounds, but said it was "still worth more than any other single stamp", The Telegraph reported Monday.

A Swedish schoolboy, who found it in 1885 among a pile of letters left by his grandparents, is believed to be the first collector of the stamp.

It is the only known misprint of an 1855 three shilling stamp that was supposed to be green. It has changed hands at least once since 1996.

US stamp expert Robert Odenweller said it was not unusual for buyers to keep details of the sale secret in the beginning and slowly release information later.

"The people who run around with that kind of change in their pockets have their own ideas about publicity," he was quoted as saying.