We've already reported on the rare mint condition 1840 Penny Black stamp - aka the world's first-ever postage stamp - appearing in The Colonial Stamp Company's philatelic auction, tomorrow (September 30).

But selling alongside it is something even more remarkable: the Penny Black's significantly rarer and more valuable sibling, the Two Penny Blue.

It was issued in the United Kingdom and Ireland on May 1840, sometime after the 1d Black, and helped pioneer the use of adhesive postage stamps which would eventually spread around the world.

The rare and valuable original Two Penny Blue stamp

While both stamps were intended to be released at the same time, there is some doubt about this. The earliest post-mark ever seen on a Two Penny Blue was stamped on May 8, 1840, two days later than the Penny Black.

According to the seller's description, the stamp's top border shows a portion of an adjoining stamp at its top (as shown in the above image).

Described as "very fine" and "a very rare mint stamp", the piece will appear for sale with an RPSL certificate and is estimated at $41,850.

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