He would have to actively assert that he was releasing it into the public domain (and IIRC, although my last discussion on the topic was ~20 years ago, such a release had not yet been tested in court).
It is now, but back then a work was public domain if released without a valid copyright notice.
Charade, a 1963 film, entered the public domain immediately on release.
https://en.wikipedia.org/wiki/Charade_(1963_film)#Public-dom...
He would have to actively assert that he was releasing it into the public domain (and IIRC, although my last discussion on the topic was ~20 years ago, such a release had not yet been tested in court).