This was the point in time when J(ames) Allen St. John was turning out mainstream assignments for magazines and advertisers, though four years prior to this cover he had illustrated the book The Face in the Pool, and it would be two years after this cover that Edgar Rice Burroughs would write his first story Dejah Thoris, Princess of Mars. I could settle this in my mind if I had access to the St. John bibliography, but I don't. Probably one of you St. John enthusiasts do (Mr. DoorTree?).
Later note: We've heard from Mr. DoorTree, and yes indeed, this cover was created by our esteemed ERBdom chronicler, J. Allen St. John. So sweet, so sweet.