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.
Feature changed title from X-5 to G-5.
It's a good bet that Blum did the art on this story as the only other artist known to have done it was Lou Fine in 1941 and his style is easily recognizable. Added by Craig Delich 2013-10-10.