Thanks to the Wikipedian who annotated this Popular Science cover noting that they had done their research. https://en.wikipedia.org/wiki/File:Popsci7.jpg#:~:text=The%20title%20was%20shortened%20to,still%20published%20today%20(2008).&text=Works%20copyrighted%20before%201964%20had,is%20in%20the%20public%20domain.

Forgetting to renew a copyright was common. Because most organizations didn't see a value except for reprints of books or for films, typically, in the time and money to track and renew. Now copyright is inherent and carries life + 75 years in the U.S. For more on that https://guides.library.cornell.edu/copyright/publicdomain