The period from 1923 to 1963 is of special interest for US copyrights: starting in 1964, works' copyrights were automatically renewed by statute; and works published before 1923 have generally fallen into the public domain. But between those dates, a renewal registration was required to prevent the expiration of copyright.  However, it's challenging to determine whether a work's registration has been renewed. 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.