I've been researching copyright law for my classical music club. I found this useful statement by the Law School of Duke University. It really cuts through the confusion surrounding U.S. copyrights.
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"The only works that are clearly in the US public domain now are those published before 1923. But what about works published after that date? Does that mean that they’re still under copyright? Well, maybe.

Citizens of the United States have to live with a frustrating lack of clarity about what older works they can use. Did the author comply with registration or renewal requirements when those were mandatory? The records are fragmentary and confused, the copyright holders hard to find.

Perhaps some post-1923 works by the authors above are in the public domain. Perhaps they are still copyrighted. We have to live in a fog of uncertainty, uncertainty that benefits no one."
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I've suspected something like this for a long time. It's good to see it verified. Next I'll put in the link.