It was less than one square inch in size and cost a penny, but it was the start of a communications revolution. The first “penny black,” a postal stamp depicting Queen Victoria’s profile, is now estimated to sell for much to £6 million at auction.

When it first went on sale in 1840, the stamp was an instant hit, enabling individuals to send a letter weighing up to half an ounce to any location in the country for a fixed fee of one cent. Over 68 million stamps were eventually sold.

Sotheby’s will auction an immaculate impression from plate 1a – the first printed sheet – in December. It is lettered A-1. The Royal Philatelic Society and the British Philatelic Association have both verified it.

According to Henry House, the head of Sotheby’s treasures auction, it is “unquestionably the most important piece of philatelic history to exist.” It’s “brimming with history” and signifies “the very beginning of social communication… allowing people to communicate from all levels of society and business to thrive.”

Communications were complicated and costly until the penny black became legal tender on May 6, 1840, and the receiver was typically compelled to pay. Sir Rowland Hill suggested a simple stamp-based pre-paid postal system.

The selected design included a drawing of Queen Victoria when she was 15 years old. Until her death in 1901, the same picture appeared on stamps for more than 60 years.

The first penny back is attached to the Wallace document, which is dated 10 April 1840 and named for Robert Wallace, a member of Parliament who chaired a postal reform committee.

It contains a Mulready stationery proof, as well as pre-stamped letter sheets and envelopes that were marketed as an alternative to adhesive stamps. The design, which included imagery of empire, was widely mocked and swiftly removed.

Alan Holyoake, a British businessman and philatelist, bought the Wallace papers ten years ago. “Isn’t the stamp lovely?” he said. “The design is a global icon, and our current Queen still wears it.”

“Before the introduction of low-cost postage, sending a letter was prohibitively expensive, so communication was limited to the privileged and wealthy. Suddenly, the postage stamp vanished. It was the gateway to widespread communication.”

Holyoake expressed regret at the sale of the penny black, but stated he planned to use the funds to start a new series.

Thanks to at The Guardian whose reporting provided the original basis for this story.