The state of Belgian Congo was established in 1908, many of its stamps were issued in Belgian Congo till 1960. When the stamp was established as the Republic of the Congo, it most famous stamp was the inverted stamp of 1894. This is a rare stamp 10f of Belgium Congo, there are around only five stamps of this series. Hence, this stamp is placed as one of the rarest stamps in the world. The other specimens have some or the other error or faults.
It is hard to find this beautiful centre inverted stamp in the philatelic market. The last know stamp was sold for 141,529 dollars.
Image courtesy: Stampauctionnetwork.com