Can't imagine that it was an Adams design. 

Right, could very well have been Infantino's design. But Adams reports that when it came to the cover designs at that time he was working "on and off" with Infantino, so it seems he had carte blanche at times (per Adams interview contained in The Krypton Companion). Infantino of course is the one who advocated for Adams to branch out to the Weisinger/Superman covers, since Adams's covers for Batman and other comics evidently resulted in higher sales for those books.

Whoever designed it, Adams reported that Weisinger "thought it was the greatest cover because it asked a question and made you buy the book. A very intelligent editorial approach. And you know, it was a boring cover to draw. I hated it. It was simple and essentially boring and it was sloppy, but it sold comic books" (same Adams interview).

EDIT-UPDATE! Ok, did a bit more research (a tad difficult to do when one is at a job, but hey, I have my priorities   ). Anyway it seems the cover "idea" was likely suggested by Cary Bates, a favorite of Weisinger's "because Cary had ideas that were his [Weisinger's] kind of ideas" (Adams again).  So Bates came up with this cover's mysterious premise, Weisinger loved it...and then Bates wrote the story fleshing it out.  

of course so!