The county agricultural agent, Hank Kimball, drives an orange Bronco with no top or rollbar. He began driving the it midway through the first season and continued driving it throughout the span of the entire series.
I was watching Green Acres the other day and saw this Bronco for the first time.
I found the above info from a comprehensive listing of Broncos in the media on one of those OTHER bronco sites, link below.
http://www.broncodriver.com/spotter.html#classic