The first gen thing never made sense to me either, but as explained to me by my son it comes from the Cummins being first offered in 89 in the "square bodies", and thus the first generation. It then bled out to all others. At least all others post 1980. Another term I don't care for as it is most commonly used in reference to the 73 to ?? GM's. The 72 through 79 being my favorite years of the D / W series.