The way it has been explained to me is this:
Each plant started at 100001 and went forward from there but they were not necessarily built in exact sequence day by day. To further complicate things, each plant built more than just one model of car so the vin numbers included a mixture of everything they produced at that factory be it Darts or Challengers. So for instance, one of my Challengers from the LA plant has a number of 100772 but that doesn’t mean it was the 772nd Challenger that factory cranked out. While you could conclude it was built very early in the production, there is no guarantee it was built after 100771 or before 100773. It might have been built days or weeks before or after a car with a vin only one number off. Safe to say though that 100772 was not built six months into the production run that year.
I did a ton of research on this exact thing for a car once, collecting pictures of hundreds of car fender tags from the same plant looking to make sense of it all and narrow down an exact build date. Everything I found fit with exactly what I just described. I found nothing outside of this that we could call an anomaly. In the end, the closest I could get that I’d be willing to make a bet on was plus or minus a week.
I think one of the tricks the pros probably rely on is looking at date codes of the other original parts on the car. In a lot of cases, parts will be dated a couple months prior to the build date of a car and it makes sense that it takes time to get all the little pieces produced and ready prior to final assembly. I would look at dates codes of as much as you could find, writing it all down and looking for common date periods. If most everything dates to December, I’d put your build date in February.
One final thought would be that since no records exist, there’s no way for someone to call you a liar. Truthfully, you could drive yourself crazy over analyzing stuff like that and who really cares if it isn’t a hemi car?
