Since nobody else answered I'll take a stab at it!!
I think????
Job #1 indicates the first part of the year. Before the 2 week summer plant shut down. & Job #2 is after that point. So J2 would be a newer truck for the same model year as J1. I didn't consider buying a J1 when I bought my 06 because J2 would have all the latest updates to the 6.0