one anecdote is that there is one slot for every continent Jeep has traveled on,
and another is that it's because there are 7 letters in America.
Another said it was for the 7 wonders of the ancient world
Or, Willys decided to make the grill 7 slots after WW2 because Ford had the trademark on the 9 slot
the real military GPW model MB had 9 slots
Chrysler now holds trademark status on the name "Jeep" and the distinctive 7-slot front grille design.
The original 9-slot grille associated with all World War II jeeps was designed by Ford for their GPW, and because it weighed less than the original "Slat Grille" of Willys (an arrangement of flat bars), was incorporated into the "standardized jeep" design.
It's because Willys couldn't trademark Ford's 9-slot design in their post-war models, so they chose a similar design that they could trademark. http://en.wikipedia.org/wiki/Willys_MB#Grille
and if you see a jeep with horizontal slots, it's a M151 by Ford. http://justacarguy.blogspot.com/2013/02/whats-explanation-of-horizontal-grill.html