A scan of the back side of the dust jacket is not available, so the lack of a copyright notice is not certain. However, at least one dust jacket featuring the same cover art, but from a different printing, is known to be published without a copyright notice.
Furthermore, front and back scans for three other Nancy Drew dust jackets are available on Wikimedia Commons, and none of them have a copyright notice.
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.
Uploaded a work by Cover art by Russell H. Tandy from [https://web.archive.org/web/20230805191209/https://www.abebooks.com/servlet/BookDetailsPL?bi=31402497280 AbeBooks] ([https://web.archive.org/web/20230805191113/https://pictures.abebooks.com/inventory/31402497280.jpg image 1 (dust jacket)], [https://web.archive.org/web/20230805191113/https://pictures.abebooks.com/inventory/31402497280_2.jpg image 2 (cover without dust jacket)], [https://web.archive.org/web/20230805191130/https://pictures.a...
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