I am not an attorney and any comments I post are not intended, nor should they be construed, as legal advice. If you need legal advice, please consult a legal expert who is familiar with the area of legal expertise you need.
It’s worth noting the U. S. copyright laws in effect during 1968 are different than the copyright laws in effect today. However, the laws in effect at the time a work was published, later creation date was added, determine copyright. For example, the law used to require putting a copyright notice in your work. That no longer applies for new works made after the laws were changed. Here are two good references or starting points:
I wasn’t aware Night of the Living Dead (the original movie) was out of copyright even though it was made after 1964. This YouTube video explains why – Why Night of the Living Dead is Public Domain: https://www.youtube.com/watch?v=L_NG1govrFo (about 7:51 minutes long).
Here’s two articles by the above YouTuber:
It gets interesting if their are musical works or other copyrighted things in a public domain item that would still be covered by copyright.
In a slightly different vein, this copyright law applies in certain circumstances to movies, games, video games, and other stuff – Studios may soon lose rights to many big 80s movies — Critical Hit. It’s how an employee was able to later acquire the rights to products he created once enough time had elapsed.