Opening a metal hatch, with Brehman, a partner in Main Line Adapt/Main Line reBuild, holding it in place, Bevin climbed in and held a pulley and rope in one hand.
"This is it," he called out: proof, so the two men believe, of the 4-foot-by-8-foot room above the ballroom in which the man who built the property, Maj. Joseph Price, reportedly hid runaway slaves.
The pulley system, accessible in a panel on the wall of the ballroom, operates a trap door in the ceiling that looks like every other panel.
"We've found that room," Brehman said, noting that it was the key that unlocked the inn's future as an adaptive reuse instead of a teardown for two more single-family houses.
In 2014, Rayer Builders, which bought the tract that includes the inn, had proposed razing the structure after determining that its restoration would cost about $2 million.
Preservationists in Lower Merion opposed that idea, citing the existence of the safe room as well as concerns about the future of other historic buildings in the township.
Enter Brehman and Mac Brand, partners with Tom Harvey in Main Line reBuild. They formed Main Line Adapt to acquire the inn for, as Brehman put it, "a respectful renovation" as three condominium units.
As Main Line reBuild, the partners have been turning buildings such as the former United Methodist Church of Narberth, the First Baptist Church of Ardmore, and the Gladwyne Methodist Church and Odd Fellows Hall into high-end condos.
The price of the inn was $615,900, records show. The price range for the condos is $700,000 to $825,000.
Work on the inn began in August, and Brehman said he hopes to be finished by February.
Secret room aside, the building - which had been carved into five apartments in the 1920s and was in abysmal shape when the partners bought it - is a treasure, Brehman and Bevin say.
Bevin, a master carpenter, pointed to a structural beam running the width of the first-floor ceiling, exposed when the old plaster was removed.
"They used an ax to trim the bottom to fit the joists into it," he said.
Instead of being tied to a ridgeboard, the roof rafters are joined with wooden pegs, Bevin said.
"I don't think I've seen anything like it," he said.
One condo will be two stories, and the two others will have three stories.
"Although it is likely to be targeted by empty-nesters, there won't be an elevator," Brehman said.
The list of specifications for renovating the inn and adding condos is extensive. Brehman said careful attention will be paid to details, including installation of recovered period flooring, extensive moldings, trims and hand-crafted features under the supervision of Bevin and other master carpenters.
"I want to reproduce the millwork and the doors as exact as can be done," Brehman said.
That may mean much of the work will be done by hand, as it was in the days when Price began his building career.
All three units will have open floor plans. Each will have a two-car garage with loft storage, basement storage, and a patio.
Despite the dilapidated condition of this piece of Lower Merion history, the William Penn Inn is structurally sound.
"The walls are 17 to 18 inches thick," Bevin said. "It's built like a castle."