According to the information from another Dodge/Cummins site, code D1 means:
SCHEDULED:
gateline schedule-scheduled to be built.
One could speculate that the date it went to a D1 code would be the date production would begin, but I guarantee nothing.
I've wondered if the longshoreman's strike on the west coast will delay the build of these trucks. Wonder if there isn't some little part needed to complete the build sitting on a ship waiting to be unloaded?