https://en.wikipedia.org/wiki/Copyright_law_of_the_United_St...:

“For works published or registered before 1978, the maximum copyright duration is 95 years from the date of publication, if copyright was renewed during the 28th year following publication. Copyright renewal has been automatic since the Copyright Renewal Act of 1992.

For works created before 1978, but not published or registered before 1978, the standard §302 copyright duration of 70 years from the author's death also applies. Prior to 1978, works had to be published or registered to receive copyright protection. Upon the effective date of the 1976 Copyright Act (which was January 1, 1978) this requirement was removed and these unpublished, unregistered works received protection. However, Congress intended to provide an incentive for these authors to publish their unpublished works. To provide that incentive, these works, if published before 2003, would not have their protection expire before 2048.”