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.
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. No record of copyright renewal for this magazine found in the Catalog of Copyright Entries. Online page scans of the Catalog of Copyright Entries, published by the US Copyright Office can be found at The Online Books Page - US Copyright Office's Catalog of Copyright Entries.
Captions
Cover of the last issue of 15 Mystery Stories, October 1950