What’s interesting to me about the above comparison is how much of John Buscema’s original composition for the cover of Conan the Barbarian #150 was cropped out of the printed version, presumably either by editor Larry Hama or by editor-in-chief Jim Shooter. Maybe if Buscema hadn’t shown Conan chopping a guy through the throat with his sword, the editors would have published it uncut. Maybe.
The only issues of Conan that I have ever searched through dusty back-issue boxes to find and add to my collection were issues in which Barry Smith inked Barry Smith, John Buscema inked John Buscema or Gil Kane inked Gil Kane. The unpublished cover of Conan the Barbarian #150 looks like pure John Buscema to me.
Here’s a youtube video that includes footage of John Buscema pencilling and then inking a pin-up of Captain America along with footage of Bill Sienkiewicz creating a quick portrait Electra with marker, brush and ink, and white-out: