The copper printing plate for the 1847 Mauritius Post Office issue will sell at David Feldman in Geneva.
It will be offered in a December auction, where it's estimated to sell for a seven figure sum.
Described in 1912 by stamp expert Alexander J Sefi as "the greatest Philatelic Treasure existing", the plate was the only one used to print the issue.
A total of 1,000 of the 1d and 2d stamps were printed, of which only 27 have made it to the present day.
They are extraordinarily valuable.
An 1847 cover displaying one of each sold for $4m at David Feldman in 1993.
Philatelist Maurice Burrus acquired the plate in 1930, hiding it away in his legendary collection.
It was recently rediscovered by his family and consigned to auction.
As a sign of the regard this item is held in, it was valued at $50,000 in 1913 - a sum equivalent to around $1.2m today.
David Feldman comments: "Handling the sale of the great MAURITIUS collection in 1993 belonging to Hiroyuki Kanai was no doubt the summit of my philatelic career, but there was always one missing item from this collection: the famous Post Office printing plate.
"What had become of the item and its location was a mystery since Maurice Burrus acquired it privately in 1930. It is at last a great honour and privilege to be able to handle this most elusive item!"
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