I found the following paragraph possibly explaining the longer number as productions numbers were just added on...
"Vehicle Serial Numbers The CJ-2A/CJ-3A Parts Book (copyright 1949) states that the beginning of production vehicle serial number is 10001 with engine serial number 10001. Vehicle serial number tags are attached with 4 screws to the engine side of the firewall near the air filter. 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. Starting in 1951, and continuing through final production, a new format was used. The prefix was changed to indicate the model and model year and the serial number itself was reset to 10001 at the beginning of each model year. See Willys-Overland Production Figures 1945-61 on the CJ-3B page for a list of serial numbers of Willys vehicles." I found it here...
http://www.cj3a.info/cj3a/date3a.htm