Works copyrighted before 1964 had to have the copyright renewed sometime in the 28th year. If the copyright was not renewed the work is in the public domain. It is best to search 6 months before and after the required year.
This image originates from the 1951 book Sam A. Lewisohn, 1884-1951, which was uploaded to archive.org, which state: "Copyright either not claimed or not renewed; item is in the public domain."
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.