The important point in this article is that the NYPL just recently digitized the relevant records, thereby making it possible to determine whether a copyright w

The important point in this article is that the NYPL just recently digitized the relevant records, thereby making it possible to determine whether a copyright was renewed. The old rule of thumb (anything after 1923 is presumptively protected) was used because looking for the absence of a renewal in a multi-decade span of printed records just wasn't feasible. The linked post in the article is very detailed and has more background: https://www.nypl.org/blog/2019/05/31/us-copyright-history-1923-1964 https://www.nypl.org/blog/2019/05/31/us-copyright-history-19...