David Mullen of Delaware City, Del., has what he thinks is a 6-foot-long piece of marble carved for the U.S. Capitol but never installed. The Architect of the Capitol’s office says it’s impossible to know for sure. (Courtesy of David Mullen)

For years, David Mullen wondered exactly what he had. And then he saw the book.

The book was “The Rise of Barack Obama by Pete Souza. There, on the cover, was a black-and-white photograph of Obama trotting up some steps on the east side of the U.S. Capitol. David’s eyes weren’t drawn to the president, but to the building itself, all those columns, curlicues and acanthus leaves.

That, he was sure, is what he had in the back yard of his Delaware house: a little bit of the U.S. Capitol.

About 1990, David bought a six-foot-long piece of marble cornice — dentils and egg-and-dart molding — from a stonemason’s near North Capitol Street. The company had worked on lots of buildings in town, including Washington National Cathedral, and was reducing its inventory before moving to Maryland.

“They didn’t know where this was from,” David said of his piece, for which he paid $300. Every time he visited Washington with his family, he would gaze at the buildings, looking for where his puzzle piece might fit.

A concrete obelisk is seen that once marked a maple tree planted on 16th Street NW in honor of a D.C. citizen killed in World War I. (John Kelly/The Washington Post)

Then he saw Souza’s book on display at a library. “I said, ‘That’s it!’ ” His piece seemed to match perfectly a part of the facade. David surmises that it could have been made as a replacement and never used.

David had thoughts of using the piece as a mantel. Instead, he said, “I have it out in my back yard as a bench, upside down, where you can sit on it.”

Now David wants to sell it. One D.C. auction house said it was too big and heavy to auction. (David thinks it weighs about 1,100 pounds.) Or he’d like to donate it to a nonprofit group, maybe the Fred W. Smith National Library at Mount Vernon.

David has no proof that it was made for the Capitol. I contacted the Architect of the Capitol’s office. Spokeswoman Eva Malecki e-mailed me back: “We have no way of knowing if this is from the Capitol or not without any documentation. It is consistent with elements in any neo-classical building.”

David thinks his chunk of marble could be valuable: “Let’s put it this way . . . a watercolor of Mount Vernon that was done by the [second] architect of the Capitol [Benjamin Henry Latrobe] sold for over $600,000. Then JFK’s leather coat sold for $600,000. Who knows where it would go, especially down in Washington, where money has a funny, different value?”

David said: “What I would like to do is find it a good home.”

If it’s unclear what David has, it’s perfectly clear what Margie and Bob Hughes have: one of the memorial markers that once stood on 16th Street NW and honored D.C. residents killed during World War I. It’s a 16-inch-tall concrete obelisk bearing a copper tag in memory of James Forsyth Diggs, a Washingtonian killed in action during the Great War.

In 1920, more than 500 such markers were placed along 16th Street next to newly planted maple trees. By 1927, many were already in disrepair. When I walked along the route in 2010, only a handful were left, most of them crumbling nubs. The rest had fallen victim to lawn mowers, automobiles, street widening and vandals.

Margie’s father, John Stevens, lived on 13th Street NW near Kalmia Road. From 1965 to 1980, he worked for the District’s Department of Sanitation. That may have been why he was on 16th Street one day and salvaged the marker. Records show that it once stood on the west side of 16th, below Tuckerman Street.

“He was always a history-type person,” Margie said of her dad, who gave her the obelisk when he sold his house in 1982. He died in 1996.

“He didn’t want it to get thrown away,” Bob said.

Bob and Margie live in Rockville. For years, the obelisk stood in their yard. Now it’s safely tucked away inside. I’m pretty sure this is the best-preserved of any of the markers.

The couple feel it should be where people can see it. I’m talking with D.C. officials to figure out where that might be. With the centennial of the Great War coming up, it’s important that tangible memories like this are preserved.

What’s in your attic?

I’m curious: Has some piece of local history found its way into your home or yard? Do you possess a bit of building, a memorial, a length of fencing or some other historic artifact? Tell me the details.

For previous columns, visit washingtonpost.com/johnkelly.