A search of U.S. copyright renewal records for 1956 and 1957 ([1], [2], [3], [4]) reveals no evidence that the then corporate heir to the corporate author of the work--RKO Teleradio Pictures--renewed copyrights to this poster or any collection of posters or any collection of material that might encompass this poster as would have been required to maintain copyright protection, if any.
There is no evidence that the work's corporate author's descendant corporation--RKO Pictures LLC--claims copyright on the image.)
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.