It helps to read what is in the U. S. Copyright site linked above.

"Therefore the U. S. copyright in any work published or copyrighted prior to January 1, 1923, has expired by operation of law, and the work has permanently fallen into the public domain in the United States." .........
"Unless the copyright law is changed again, no works under protection on January 1, 1999, will fall into the public domain in the United States until January 1, 2019."

"If a work was first published or copyrighted between January 1, 1923, and december 31, 1949, it is important to determine whether the copyright was renewed during the last (28th) year of the first term of the copyright. This can be done by searching the Copyright Office records or catalogs....." If no renewal registration was made, copyright protection expired permanently at the end of the 28th year of the year date it was first secured."

A re-writted or revised work, of course is another matter.