Released in 2008 and funded by Hewlett Foundation. From front page:
... This database makes searchable the copyright renewal records received by the US Copyright Office between 1950 and 1992 for books published in the US between 1923 and 1963. Note that the database includes ONLY US Class A (book) renewals.
The period from 1923-1963 is of special interest for US copyrights, as works published after January 1, 1964 had their copyrights automatically renewed by statute, and works published before 1923 have generally fallen into the public domain. Between those dates, a renewal registration was required to prevent the expiration of copyright, however determining whether a work's registration has been renewed is a challenge. Renewals received by the Copyright Office after 1977 are searchable in an online database, but renewals received between 1950 and 1977 were announced and distributed only in a semi-annual print publication. The Copyright Office does not have a machine-searchable source for this renewal information, and the only public access is through the card catalog in their DC offices.
Stanford has performed two rounds of testing in order to assess the accuracy of this database. In each round, we pulled a minimum of 500 book titles published in the US between 1923 and 1963 from the Stanford library catalog. The works were checked manually in the CCE, and, in the first round, a subset of 100 records was also sent to the Copyright Office to be checked by their in-house staff. Each of these items was then separately searched by project staff in the Copyright Renewals Database. In each round, the error rate for the database was found to be less than 1%, although in practice there is significant opportunity for user error or other problems in searching. Details of these issues can be found in Stanford's final report to Hewlett on the project ( PDF), and in the two search results sets ( Search Set #1, Search Set #2).
No license specified but state on home page:
The full data set for the Copyright Renewals Database is available for download here. Stanford welcomes reuse of the data in other systems and search tools.
Given that original data is public domain (renewal records published by copyright office) assume that this material is public domain.