The Presbyterian Historical Society at Fifth and Lombard looks like any number of 18th-century landmarks that dot Society Hill. The rigorously symmetrical three-story brick building is capped with a gabled pediment and features the same dentil moldings you might find on Pennsylvania Hospital or Independence Hall. The difference is that the historical society was built two centuries later, in 1967.

That makes the Georgian-style replica an unusual exception to the philosophy that guided Society Hill's urban renewal. When the city decided to reconstruct the neighborhood in the late 1950s, it systematically purged it of anything that wasn't from the colonial period or the early 19th century. The plan was to cleanse the area of its later architecture and emphasize its authentic colonial past.  New infill buildings were allowed, but architects were encouraged to design them in the reigning modernist style so that the colonial survivors would stand out clearly from the new construction. Some replicas were created, but they were based on actual colonial buildings that had deteriorated. Maybe that's why I've always assumed the historical society was one of those 18th-century relics.

The Presbyterians had other ideas, it seems, and preferred to look back in history for their inspiration. After giving up their lavishly decorated high-rise on Walnut Street, the Witherspoon building, they acquired a large plot of land next to the colonial-era Presbyterian church now known as Old Pine to house the historical society. Rather than choose a modernist architect, the church sought out a well-known traditionalist, G. Edwin Brumbaugh.

The son of a Pennsylvania governor, Brumbaugh was an early preservationist who committed himself to painstaking restorations of colonial architecture, including Grumblethorpe in Germantown and Old Swedes' Church in South Philadelphia. He took a rigorous approach to restoration, measuring proportions and analyzing paint samples. Although Brumbaugh didn't design many new buildings, those he did build were faithful replicas, like the tiny Dilworth House on Washington Square.

At the historical society, you can see his attention to detail in the graciousness of the lobby, which is fitted out with a checkerboard floor and elaborate wainscoting, and painted the exact shade of robin's egg blue found in colonial homes. Brumbaugh's meticulous approach is in sharp contrast to that of another historic pretender, the new Museum of the American Revolution at Third and Chestnut, designed by Robert A.M. Stern Architects. Though it borrows bits and pieces of the style to evoke a Georgian building, no one would ever be fooled.

Although Brumbaugh was absolutely faithful to the tenets of  colonial design, he also made sure the building could function in the modern world. The building houses the church's entire archive, dating to the arrival of the first Presbyterians in the New World. Because the Presbyterians are "fanatical about keeping records," said marketing director Fred Tangeman, the archive has been a boon to all kinds of researchers.  Brumbaugh installed climate-controlled storage in the basement. In 1977, it was expanded, and the enormous archive now runs the entire length of the block, continuing below the Old Pine Community Center next door.

Given the Presbyterians' focus on the past, it is fitting that the historical society building has now reached age 50, which makes it an architectural antique, according to preservation standards. The historical society is marking the milestone with a major exhibit, "Presbyterians and the American Revolution." Opening April 24, the show will run through October 2018.

The historical society is open from  8:30 a.m. to 4:30 p.m. Monday through Friday. Admission is free.