King said a rifle of this kind could fetch tens of thousands of dollars. But because it is connected to a significant piece of history, it is priceless.
"This is a really valuable artifact," King said. "It has a documented connection."
John Brown led the raid in Harpers Ferry to protest slavery.
In 1858, he held a secret meeting in Chatham, Ontario, to ratify a "provisional constitution" for a United States free of slavery.
A year later, he met Shields Green and Frederick Douglass in a Chambersburg, Pa., quarry to go over his plan - capture the U.S. Armory and Arsenal in Harpers Ferry.
The attempt ended badly. While Brown's raiders held armory workers hostage, local militia cut off escape routes and the Marines poured into town.
Brown and four others were captured and taken to jail in Charles Town less than 36 hours after the raid began. Brown was executed on Dec. 2, 1859 for conspiring with slaves.
Some people say the outcome of the raid was the spark that ignited the Civil War, King said.
"In a sense, you could say this gun fired the first shot of the Civil War," King said.
At the time of his capture, Brown's rifle, called a sharps carbine, was taken from him and given to the governor of Virginia as a souvenir.
That rifle was passed to a friend of the governor's, who gave it to another friend, who shipped it to his cousin, Henry Lansing Wardwell, in 1870.
About 100 years later, Wardwell's great-grandchildren found the rifle with the letter. At first, the family wasn't aware how significant the rifle was.
"(My great-grandfather) had a lot of historical things in that house," said Florence Arwade. "For me, it was another thing he had collected."
"Since then it's been pretty much wrapped in a sheet in my closet," said Rick Carnes, Wardwell's great-great-grandson. "We wanted to protect it mostly."
But his wife, Susan, realized the rifle's value. She wasn't comfortable with the gun in the house for fear something would happen to it. She called some national parks, which led the rifle back to Harpers Ferry, Carnes said.
"I think she's really happy now," Carnes said.
The rifle has been on loan to the park by Wardwell's family for five years. Park officials say it will be displayed soon in the John Brown Museum.
The park draws about 500,000 visitors each year, King said.