In a bizarre, early-morning burglary in October 1971, a thief used a crowbar to break into an “unbreakable” case at Valley Forge National Historical Park and left with a rifle that dates back to the American Revolution.

The whereabouts of the filched firearm — one of the few surviving works of Pennsylvania master gunmaker John Christian Oerter — remained a mystery for nearly 50 years.

Around the same time, other antique weapons were stolen from nearby museums — an 1830s Kentucky rifle stolen from the Historical Society of York County, a Colt Model 1861 percussion revolver taken from the American Swedish History Museum in FDR Park, a C.S. Pettengill double-action Army revolver removed from the Hershey Museum.

The weapons finally turned up in July 2018, when they were sold by Thomas Gavin, who pleaded guilty Tuesday in federal court in Philadelphia to stealing and selling the artifacts. Gavin, 78, was ordered to be held ahead of a sentencing hearing scheduled for Nov. 15, with bail set at $100,000. He faces up to 10 years in prison.

According to the plea agreement, Gavin sold two antique rifles, a trunk filled with more than 20 antique pistols, and a Native American silver conch belt at his home in Pottstown in July 2018 to antiques dealer Kelly Kinzle for $27,150.

After examining the rifle and realizing its historical significance, Kinzle reached out to his attorneys, who connected the weapon to the theft at Valley Forge. He handed it over to investigators, and the rifle was eventually returned to the Pennsylvania Society of Sons of the Revolution, the organization that donated it to Valley Forge for display in 1963.

The rifle is now housed in the Benninghoff Collection at the Museum of the American Revolution in Philadelphia.

“I actually thought it was a reproduction,” Kinzle told The Inquirer in 2019. “My first inclination was that it had to be fake, because the real gun isn’t going to show up in a barn in today’s world. Things like that are already in collections.”

Gavin was questioned by FBI agents and detectives from the Upper Merion Township Police Department in February 2020, according to the plea agreement, and admitted he stole the Oerter rifle and other antique weapons from museums across Pennsylvania in the late 1960s and early 1970s.

Gavin’s attorney declined to comment Wednesday.

It was a Boy Scout on a tour of Valley Forge who noticed the rifle was missing in 1971, The Inquirer reported at the time. Authorities waited nearly a month to disclose the heist out of fear the thief might panic when he saw the antique’s value and discard it.

The rifle is one of just two dated and signed by Oerter known to still exist. The other was given to the future King George IV in the early 1800s by a British cavalry officer who served in the war. It’s housed in the Royal Collection at Windsor Castle.