Published in August 1923, this “Scientific Fiction Number” is considered to be the first American science fiction magazine. The success of this issue, and the one the following year, led Gernsback to publish a specialized fiction magazine, Amazing Stories, beginning in 1926.
This looks like a sci-fi retelling of Perseus. Note Medusa’s head and the guys with guns.
This entire issue can be downloaded here
Warning: Past May Contain Dinosaurs
Also includes Race Around The Moon, and a prize for the best story written about this cover, presumably because The Warning From The Past wasn’t very good, although it wasn’t the first time someone ran this contest.
Also includes Madness From Mars and The Jules Verne Express, which was actually a real thing in the 1980s
This cover story was later republished in Startling Stories, July 1949
Also includes The Dark Age and The Infinite Enemy