The item has no copyright markings on it as can be seen in the links above and in the orginal uploaded copy. At bottom of the uncropped copy is Country of origin USA, Morgan Litho Company, Cleveland, and a number which seems to pertain to the printing of the poster.
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 (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.
{{Information |Description=Poster from the 1938 film ''Riders of the Black Hills''. |Source=[http://www.ebay.com/itm/Riders-of-the-Black-Hills-One-Sheet-Movie-Poster-The-Three-Mesquiteers-1938-/201187352945?pt=LH_DefaultDomain_0&hash=item2ed7b36171&nma...