Some info verified using a DCM scan (http://digitalcomicmuseum.com/)
The on-sale date is from the publication date recorded in the Catalog of Copyright Entries, Part 2, Periodicals, 1940, New Series, Vol. 35, No. 4.
Title refers to a movie adaptation of "Seven Keys to Baldpate", a play by George M. Cohan (1913), adapted from a novel of same year by Earl Derr Biggers (Charlie Chan's author).
The movie is likely 1935 version: http://www.imdb.com/title/tt0026978/