Interesting! I'd never seen those before. Honestly, they look more like Kane prelims.
Yeah, they are kinda unusual. Originally, I found the second piece of concept art in the 'Frazetta sketchbook volume 2' by Vanguard publishing. Unlike the above artwork the punch-holes were cropped. In the sketchbook, on the opposite page there is another very similar concept illustration. I will try to find it online, if there is no joy, I'll scan the artwork on this thread later.
Apparently these cover concept designs were rejected because they wanted to see Conan's face on the cover.