I distinctly remember the fright that monster gave me as a six year-old kid!
First, the awesome and memorable cover painting of the alien from "Invasion of the Saucer Men, by none other than Ron Cobb!
(Click on images to enlarge; once open you may have to click on it again to view full-size.)
Bonus: from the same issue, the scary giant from "Lost In Space." Since Chambers did the makeup effects for this show, it's likely he had a hand in making this creature also. Here's an earlier post I did on the model of this monster.
Bonus: from the back cover, a full-page ad for the "Forgotten Prisoner of Castlemare." You can see a whole post I did on this model here.