Staff of the State Library in the 1930s through the early 1960s indexed the family surnames mentioned in the titles in the county and family history collection. This index card file was held in the Genealogy/Local History spaces of the State Library.
These files are now on 21 reels of microfilm in the State Library Genealogy Reading Room [929.20973 Surname Files]. They are also available digitally in the State Library's genealogy resources in the PA Photos and Documents collection at the Power Library Website.