Until this week, the Library of Congress didn't know the identity of young Confederate soldier shown in a photo it received last year. The photo was included in an ad spotted by Karen Thatcher. (Courtesy of Library on Congress)

The old photograph shows a young Confederate soldier posing proudly in an elegant uniform, with a pistol in his belt and a saber in his hand.

It is a well-known 1860s ambrotype worth thousands of dollars, and experts had identified the rare style of his buckle, the make of his English revolver and the cavalry outfit in which he served.

But scholars at the Library of Congress, which was given the photo last year, had no idea who he was. Like scores of Civil War portraits, his was listed as “unidentified.”

Until this week.

Last Sunday, Karen Thatcher of Martinsburg, W.Va., opened a Washington Post Civil War history supplement. She spotted the picture in a Library of Congress advertisement, and realized: “That’s Uncle Dave!”

In an instant, for posterity, the soldier was given back his name — and his story.

He was David M. Thatcher, a farmer’s son from Martinsburg, who enlisted at 17 in Company B, Berkeley Troop, First Virginia Cavalry, on April 19, 1861, a week after the war began.

At 19, he was mortally wounded in battle outside Warrenton in 1863, and he was buried in the cemetery at Martinsburg’s Tuscarora Presbyterian Church, where he rests today. Family lore has it that his parents brought his body home with a horse and wagon.

The identification thrilled Karen Thatcher, a retired federal government worker, as well as the library and the collector, Tom Liljenquist, who bought the picture several years ago and donated it in October.

“We’re just tickled to death,” Thatcher said in a telephone interview Wednesday. “There’s something very satisfying about this 19-year-old boy who died in 1863 who was [listed as] unidentified . . . that we’re able to put a name to that face.”

Liljenquist, who has given the library almost 1,000 Civil War portraits in recent months, said: “I’m just awestruck. . . . This anonymous young boy has gotten his life back.”

The identification was made when Thatcher saw that the photo in the advertisement looked almost exactly like a larger image she had of David Thatcher, an ancestor of her husband, Larry.

The larger image — which was probably copied from the photograph — is a “crayon enlargement,” said Carol Johnson, the Library of Congress’s curator of photographs.

Crayon enlargement was a common 19th-century technique in which a picture was enlarged, printed and then colored in with charcoal or chalk to make a bigger portrait.

“That way, people would have something they could hang on their walls,” she said. “Since he died in the war, they probably had this made . . . as a way to remember him.”

What happened to the original photograph is less clear. “Maybe he gave it to his girlfriend before he left for the war,” Johnson said.

Eventually, someone came into possession of it and didn’t know who he was, and it went onto the collectors market.

Johnson said experts were able to glean some information about the soldier from his uniform type and accouterments — his frock coat with the lateral, braided “frogging,” his Virginia state seal belt buckle, and the crossed sabers and the number 1 on his cap.

His unit was deduced because his get-up exactly matched that in another picture of a soldier from Berkeley Troop, the experts said.

But Thatcher’s name remained elusive.

Liljenquist, of McLean, said he bought the picture years ago at a Civil War show, probably in Virginia. “It’s a well-known photograph,” he said. “It’s been published in a few books.” But no name was associated with it.

David Thatcher, it turns out, served in a storied unit that was originally commanded by the South’s legendary cavalry general J.E.B. Stuart.

David Thatcher was wounded Oct. 19, 1863, in the Battle of Buckland Mills, which was such a complete Confederate victory that the rebels called it “the Buckland Races.” He died the next day, acccording to Johnson’s research.

His tombstone reads:

When thou goest out to battle

against thine enemies, be not

afraid of them, for the Lord

thy God is with thee.

Karen Thatcher said that the Civil War is still “close” in her area and that her family, with deep roots there, has long known of the story.

“If you have a family member who dies at the age of 19 in the Civil War, everyone knows that,” she said. “And this picture was just always in the family. And so you just knew that that’s who it was.”

“My husband jokingly calls him ‘Uncle Dave,’ ” she said of the soldier, who was three generations removed — a brother of her husband’s great-grandfather.

She said she and her husband have a small prewar photograph of David Thatcher in civilian clothes attached to a certificate honoring his memory. That photo, too, resembles the other images.

She said their “crayon enlargement” was a copy of one that had been in her husband’s household when he grew up and was passed down to one of his nieces.

“It looks like a drawing of a photograph,” she said.

A history lover, Karen Thatcher said she opened The Post’s Civil War section, and staring back was an identical copy of the picture that the niece had given them.

“Except, I could tell that it was a photograph . . . not a drawing of a photograph. I thought, ‘Son of a gun.’ I thought, ‘Gee whiz.’ I thought, ‘Isn’t this amazing?’ ”

She said she went to the library’s online gallery, and “there’s Uncle Dave.”

She called the Library of Congress on Monday morning.

The library said Friday that it had already changed its master catalogue to add Thatcher’s name to the the picture’s description, and remove the word “unidentified.”

The library said it would make the change to its online gallery next week.

Read more of The Washington Post’s Civil War coverage:

The Monitor’s enduring fascination

A slave’s audacious escape

Second Manassas showed how bloody war would be

Civil War blog: A House Divided