
A cover with a rare $2 stamp on it is expected to fetch up to HK$6 million in an auction this Sunday.
The Chinese stamp is one of only 50 issued with an image of the Peking Hall of Classics mistakenly inverted.
While between 40 and 50 of the stamps have been found, only one cover has been found with the stamp used on it which is why it is so valuable, Neill Granger, a stamp specialist with auctioneer Spink, said.
"It was a mistake by the printers," he said.
"When they printed it they put it in upside down."
The story of the $2 stamp went back to when the Republic of China was founded in 1912.
The government needed to issue a new set of stamps and asked one of the best printers of engraved stamps at that time, Waterlow and Sons in Britain, to produce the new set.