Just to continue the history a bit - Ford got a contract in 1951 to design a replacement for the M38 series and produced the M151, a completely new design. It had an independent suspension, for example. The M151 grille has horizontal slots because Willys / Jeep now owned the horizontal slot look.
The HUMMWV was designed by AM General, which was spun off from American Motors. Because the Jeep intellectual property passed through AMC, the HUMMWV (and its civilian offshoots) had the seven slot grill as AM General had rights to the design.
Because the Humvee was in civilian production, that *might* be why the JK has the "Jeep" logo on the front where all other models of CJ and Wrangler do not- as there was another vehicle with a seven slot grille in production at the time. That last bit is just speculation on my part though.