Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (70 years p.m.a.), Mainland China (50 years p.m.a., not Hong Kong or Macao), Germany (70 years p.m.a.), Mexico (100 years p.m.a.), Switzerland (70 years p.m.a.), and other countries with individual treaties.
Note that even though the publisher's copyright was not renewed, stories and nonfiction pieces published in Famous Fantastic Mysteries may have had their copyrights renewed by their authors, or may be reprints subject to prior copyrights.
Captions
Cover of Famous Fantastic Mysteries, June 1953, including a reprint of Kafka's The Metamorphosis