The principal issue here is whether or not the person who purchased the rights to the 1964 movie, also purchased the rights to the 1939 story. The secondary issue is whether or not the 1939 story has entered the public domain, due to an expired copyright.
By example, if the story's copyright was originally published with a copyright notice (probably true), but not renewed, then the copyright has expired -- which means that you can do whatever you want with the story line (though not with the movie characters -- which based upon your facts, is not at issue).
If I were representing you, I would probably ask the person claiming copyright infringement to show proof that the original copyright was licensed from the 1939 published and/or author, and that the copyright was renewed before it expired ( circa. 1967).
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