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An extremely rare copy of the Declaration of Independence that was hidden behind wallpaper to protect it from ransacking Civil War troops, has surfaced.

The Washington Post reports that the copy, which is one of only 51 known to exist, resurfaced last month after its purchase by billionaire David M. Rubenstein, co-founder of private equity firm The Carlyle Group.

Rare document dealer Seth Kaller, who worked on the sale, told Fox News that the calfskin copy is one of two that were given to founding father James Madison by Congress in 1824.

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“It’s extremely rare to find one with a close connection to a founding father,” he said, adding that the location of the second copy is unknown.

Madison likely gave the existing copy to his nephew Robert Lewis Madison, with Kaller noting that the document was not part of the Madison estate when the founding father died in 1836.

By the time of the Civil War, Madison’s son, Robert Lewis Madison Jr., was in possession of the Declaration. Madison Jr., a doctor who served in the Confederate army and was a physician to Robert E. Lee, was apparently concerned that ransacking troops would destroy the document. According to a 1913 newspaper article, the doctor’s wife hid the copy behind paper on a wall at the family’s home, which was likely in Lexington, Virginia.

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The historic artifact survived the war, albeit with some damage from moisture, and remained in the Madison family. The Washington Post reports that Madison’s descendant Michael O’Mara found the copy when he was going through family papers following his mother’s death in 2014. O’Mara’s mother, Helen, was the great granddaughter of Robert Lewis Madison Jr.

The Declaration, in a broken frame, had been in a cardboard box at O’Mara’s office outside Houston for 10 years prior to its re-discovery, according to the Washington Post. Before that, the document was in O’Mara’s parents’ home in Louisville, Kentucky. Initially displayed on a mantelpiece, the document was reportedly stored in a bedroom closet when the frame cracked.

Last year, experts from Washington Conservation Studio spent about 10 months working on the moisture-damaged document.

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Rubenstein, who owns five copies of the document including the latest version, told the Washington Post that he paid “seven figures” for the Declaration.

The Declaration is one of around 200 made in the early 19th century amid concerns about the condition of the original 1776 document. In 1820, then Secretary of State John Quincy Adams commissioned printer William J. Stone to make an exact copy of the Declaration. Stone subsequently engraved the copy on a copper plate and printed it on calfskin.

Stone worked on the copy for nearly three years – as a result his engraving is the best representation of the Declaration as the manuscript looked at that time, according to Kaller.

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In 1823, the State Department requested 200 copies from the facsimile of the engraved plate. Stone printed 201 copies, one of which he kept for himself. Stone made an additional set of copies in 1833.

Set against this backdrop, the latest find is extremely significant, Keller told Fox News. “We get about five to 10 calls a week from someone who thinks that they have one of these – if it’s not one of these really rare early first or second editions printed by William Stone, it’s not valuable.”

The recently resurfaced copy will be on display at the Smithsonian Nation Museum of American History.

Follow James Rogers on Twitter @jamesjrogers