This 1978 Wards page explains a good deal as to why the Venus Space Probe from the Kenner Six Million Dollar Man line is so tough to find.
The one obvious reason is the line had run out of steam and buyers were now ordering lightly. The other reason might be that the Venus Space Probe was actually marketed all by it’s lonesome. It’s the sole SMDM piece in this Ward’s catalog, what fun is it, if you can’t get a doll to go with it? We all know the rest of the story, this piece had a limited run and is ungodly rare. I don’t collect Steve Austin and crew but I find the “Grail” in any line oddly attractive…..