Before 1976, copyright protection was a lot different than it is today. Material published with a © notice and registered with the U.S. Copyright Office between 1923 and 1963 were only granted 28 years of copyright protection with an optional 28 year renewal. Determining whether these materials have entered the public domain due to a lack of renewal has been difficult for researchers and libraries looking to use or make available aging content. Records of renewal were initially published in annual or semi-annual print publications, or recorded on cards held in a catalog at the U.S. Copyright Office. Previous efforts to make these records more accessible include a digitization project of the Internet Archive that made the print volumes available online, but we were still lacking the ability to search across volumes.
In March of 2018 The New York Public Library announced a project to extract data from this collection of records and put it into a searchable database. That project is going strong and uncovering a wealth of material that has fallen into the public domain due to copyright expiration. Find out more here.