Originally they were installed in the "inner" positions on the front and the "outer" positions on the rear. Since the four cylinder Js did not have them, restorers often liked to add them and sometimes put them in the other positions. I've seen restorers do them alternately too and one even told me he put the rear ones in the inner holes because "they'd provide more protection" in those postions. In my opinion, those cheap little guards would likely provide no protection in a collision. There was also a full page spread advertisement for the Henry J in either Life or Sat Evening Post which had a picture (hand rendered) of a green '51 Henry J where the front guards appear in the outer positions. Since it's an artist rendering, and the other holes (with the bumper bolts) aren't even included in the drawing, I would say the artist was in error but that ad may have confused some restorers / owners too.
Based on actual photos (not artist renderings such as the brochures and some sales/advertising literature) in KF Dealer News and the shop manual, I would conclude that, yes, the front ones should be installed in the inner holes and rear ones in the outer holes in order to be truly authentic.
