United States copyright notice on high resolution scans of the poster: Allied Artists, 1958. Marking: 58564. An online search at the US copyright office did not find that the copyright was renewed. In the absence of renewal of the US copyright, this poster art entered the public domain in 1987.
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.