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 said.
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 US dollars) price it set a record for in 1996.
"It is still worth more than any other single stamp" including the even older Two Penny Post Office Mauritius Blue that sold for 1.5 million francs (then 1.4 million US dollars) in 1993, said Mr 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 US stamp expert Robert Odenweller said it was not 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," Mr Odenweller said.