English: Copy of the front cover of the Big Little Book Jack Armstrong and the Ivory Treasure.
Since General Mills' Wheaties brand was the sponsor of Jack Armstrong, it appears that this Big Little Book was specially prepared as a radio premium for the company.
To make certain this is in the public domain, renewals in drama (radio script), artwork (cover art) and books were checked for the years 1964 and 1965. There were no listings for the title nor for anything by the writer or artist of the book cover. General Mills also had no renewal listings in these categories. There's no evidence of a continuing copyright.
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.