There is one last (?) avenue concerning the Public Domain status of a work purportedly under copyright that I have not mentioned yet. Namely, whether the work was officially registered with the U.S. Copyright Office -- a requirement for all copyrighted work before the United States signed onto the Berne Convention in
1989.
The workflow for copyright research of works with a proper copyright notice (published in the United States) is, as far as I understand presently:
1) For works published between January 1, 1924 and January 1, 1964 --
a. assume they were registered, and
a. determine the year given in the copyright notice for the specific edition in question
b. determine whether the work was in fact registered with the U.S. Copyright Office.
3) For works published between March 1, 1989 and March 1, 1994 --
a. determine whether the work was in fact registered with the U.S. Copyright Office.
Clearly, this affects many of the works that have to date been set aside due to reasonable concerns. The important take away for those working to make these works more broadly accessible in new digital editions available for adaptive re-use is: the status of any given copyright is only attainable through objective research.
Aharon