Because anything could have happened to these vehicles in the 50 years since they were manufactured, a large sampling of vehicles is necessary to determine which assumptions are valid.
From the information I have collected to date the following differences are appearing.
Willys Service Bulletin 53-5 explains "the effective serial numbers given apply to changes made in vehicles assembled at the Toledo plant and do not apply to those assembled in our west coast plant".
The CJ-3As military sibling, the M38, also went through many changes.
This information is preliminary and it changes rapidly as new information is received from enthusiasts like you.
It makes sense, but before you proudly take a selfie next to your cool VW GTI, the study says profiles with hatchback photos see a It probably goes without saying that you should take all this with a grain of salt.
Later in 1949, and for the remainder of production, the body tag was eliminated. The body tag to the left is from a 1949 CJ-3A, thanks to Andy Testo for the photo.
Body tag numbers are initially close to the vehicle serial number and then skew to about 1200 less than the vehicle serial number. Early in the 1949 model year Willys began to build their own bodies.
The vehicle serial number had a prefix of CJ-3A for the first 2 years of production.
During this time the serial numbers continued to climb with no regard for model year.