Assuming that your 1951 Willys is a Universal, not a pickup or station wagon, it is a CJ-3A. It was made from 1949 to 1953. The previous model was the CJ-2A. The CJ-3A was succeeded in 1952/1953 by the CJ-3B. The military version was the M-38.

If original, it should have a 4-cylinder L-head engine. The later CJ-3B was developed to contain a slightly larger 4-cylinder F-head engine.

You might want to visit the following web site for more info: http://www.film.queensu.ca/cj3b/CJ4Prototypes.html

Good luck with your new acquisition.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.