Very rare. It would have to be an early WWII Jeep. Ford built the same Jeep for the war as Willys but after a while the Army told Ford to take the name off of them because they considered it to be advertising.
The quickie version of Jeep history is that in the late 1930's the Army asked a number of car companies to come up with a 4wd utility vehicle. The winning proposal came from the Bantam Car Company in Butler, PA, not far from me actually. About that time WWII was ramping up and the Army figured out that Bantam was too small a company to make vehicles in the numbers needed, so they forced Bantam to give their blueprints to Ford and Willys also. Identical Jeeps were made by Ford and Willys, the early Ford models had the Ford script stamped in the tailgate. The oldest Jeep in existence is one of the original 30-some prototypes made by Bantam and is on display in the lobby of the Sen. John Heinz History Center in Pittsburgh, PA, on loan from the Smithsonian. I've seen it.
http://www.oramagazine.com/pastissues/0501-issue/050113d-old-iron.html