I should have looked it up before I posted
The rule is 95 years. Works created and copyrighted (that is, registered or published) prior to January 1, 1978, are protected for 95 years from the date the copyright was originally secured (95 years from the earlier of the registration or publication). The 95-year period is divided into an initial term of 28 years and a renewal term of 67 years.1 Works created prior to January 1, 1978, that were neither copyrighted nor fell into the public domain before that date are protected for the life of the author plus 70 years, provided (i) in no event shall the term of the copyright in such a work expire before December 31, 2002, and (ii) if such a work is published on or before December 31, 2002, the term of the copyright shall not expire before December 31, 2047.
Source: https://www.royaltyexchange.com/length-of-ownership-music