Just an FYI--this is a pretty interesting history that I was only vaguely
aware of.

https://help.hathitrust.universityofcalifornia.edu/support/solutions/articles/9000220606-how-uc-s-hathtrust-collections-were-built

Also this paragraph really drives home the effects of previous
registration/renewal formalities on copyright status.

In 2015, Google introduced new candidate lists to include volumes beyond
those determined to be public domain based on their publication date. These
included selected volumes from the US Renewal Era (1923-1963), which may
not have had a copyright registered or renewed, US Federal government
publications, US state government documents (from states where such
materials are public domain), and various international government
documents. *Google’s decision to include targeted volumes from the Renewal
Era was an astute one: a 2019 study from the New York Public Library
estimates that 75% of volumes published in the US Renewal Era may be public
domain due to lack of registration and/or required renewal*.

-- 
Timothy Vollmer
Scholarly Communication & Copyright Librarian
University of California, Berkeley
Doe Library, 189 Annex
Berkeley, CA 94720
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Pronouns: he/him