The first three ads appeared in Popular Mechanics during 1946, first in February, then March, then June. Note that the February ad does not include the King of Jeeps moniker, but the later two ads have it.

February 1946 Ad in Popular Mechanics, the earliest one I could find.

By June 1946 Berg was advertising Amphibious vehicles.
A November 1955 ad highlighted the use of private labeling by Berg. The hardtop looks like a Worman Jee-Cab that’s been rebranded “the Jeep King All Steel Cab”. I’m sure the “Jeep King Snow Plow” is another private labeled product.
This was the latest Ad from Berg that I found, though my searches were not extensive.