These records are quite varied, consisting of a number of different formats. Originally kept as “vertical files” the Historical Society of Pennsylvania has maintained this collection of “Old Philadelphia Families Correspondence and Genealogical Data” for many years.
You can expect to find vital information, namely birth, marriage, and death, alongside other snippets of history including burial locations, newspaper clippings about the individual or family, and documentation such as images of original birth and marriage certificates. Anyone with deep Pennsylvania roots will want to explore this material at length.
The “Old Philadelphians” carry well-established lineages. Most families descend from English, Welsh, or German origins. Anglo-Saxon Protestants in the Episcopalian Church or the Society of Friends dominate the known population.
A fairly comprehensive list of associated surnames was established in 1963 by Nathaniel Burt. Anyone with these surnames in their family tree should certainly be investigating materials related to the Old Families.