The head gaskets are pretty much the same on all Job of 08 truck. Yeah, the later ones are supposedly better, and supposedly thicker or whatever, but I've yet to see a head gasket fail on a 6.4, job 1, 2 or 3. Part number updates can occur for any number of reasons, including packaging changes, or what comes in the box, or an updated part that comes in the box.
The job 1 trucks, specifically those built before 8/20/2007, had the early, un-covered HPFP gasket with the loose wires that would rub through on the pump, chafe, and cause a no start and possibly a blown fuse #74 in the BJB. Cab removal required to repair this concern.
There is a RAFT of TSB's with date specific ranges on them, but most interesting I find the Module Reprogramming TSB's.
Prior to 8-2-2007
Between 8-3-2007 and 1-2-2008
Between 1-3-2008 and 9-15-2008
Looking at those dates, would make me think that those are the SPECIFIC breakdown dates for jobs 1, 2, and 3, since if you think about it, more than likely 2008 model year production ended in September 2008, when the 2009's started rolling off the line.
The TSB's with the dates on them are calibration updates, which took the PCM
and TCM to the same calibration level, so I'm unsure why they broke them into seperate build categories. Unless it has something to do with WHAT changes each truck will have, and be noticed by the customer, depending on what calibration was in the truck when it was purchased new.
That's all I care to contribute at this time, since it's 4:22am, and I haven't slept yet. I work at 7. *sigh*