Technically, that's not a "jet-pack" in the sense of being able to lift you off the ground at normal earth gravity. It's really more of a prototype MMU.
So in fairness, PM actually did get this one right...
I happened to come across an issue in Sept. 1961 doing a search, thumbed through and found an article about President Kennedy's new specially created Lincoln with a three-part retractable top. Sounds much better before you know how the president met his end ...
So in fairness, PM actually did get this one right...