Science Fantasy and outright Science Fiction elements. This wasn't universal, of course, and probably was more common in the wilds of California, but it still came out before we had any idea of a cosmology (and frankly, in a lot of cases had already largely rolled our own--most of the demons didn't come out that early, after all (balrogs and hellhounds were about it as I recall--the rest came out in Eldritch Wizardry if I recall correctly). But in the day you'd still see things like various SF originated monsters and sometimes classes, and various bits technology as "magical" treasure (and some characters had seen it often enough to know the difference, even if they didn't thoroughly understand it).
It was fuzzy right carrying along. The original Witch World books showed that, as did things like The Empire of the East books. Not even counting things like the surviving Planetary Romance novels or things like the World of Tiers that Nevin mentioned above.