Geneva ? The Swedish “Treskilling Yellow” retained its title as the world’s most expensive stamp when it changed hands at a private sale shrouded in secrecy, the auctioneer claimed Saturday.

The one-of-a-kind 1855 misprint was sold to a group of buyers who asked that their identities and the winning bid be kept confidential, said auctioneer David Feldman.

He declined to reveal whether the sale matched the 2.875 million Swiss francs (then about $2.3 million) price it set a record for in 1996.

“It is still worth more than any other single stamp” including the even older “2 Penny Post Office Mauritius Blue” that sold for 1.5 million francs (then $1.4 million) in 1993, said Feldman.

He added that both the price and identity of the buyers, who took part in the telephone auction against a single rival bidder, would likely become public knowledge eventually.

Noted U.S. stamp expert Robert Odenweller said it wasn’t unusual for buyers of such valuable items to keep details of the sale secret at first, only to release information bit by bit later.

“The people who run around with that kind of change in their pockets have their own ideas about publicity,” Odenweller told The Associated Press.

For years the owner of another unique stamp, the 1856 “British Guiana 1 cent Magenta,” remained a mystery until it transpired that it had been bought for nearly $1 million by chemicals fortune heir John du Pont in 1980, Odenweller said. That stamp is believed to lie in a bank vault while du Pont serves a 13- to 30-year sentence for third-degree murder.

The Treskilling Yellow is the only known misprint of an 1855 three shilling stamp that was supposed to be green. It has fabled status among collectors and is considered one of the world’s most valuable objects for its size.

The auction house valued the stamp at $1.87 million to $2.5 million before the sale, but actual sales prices can vary greatly depending on what the buyers feel such unique items are worth.