how to decode the sequence number

The date on the fender tag is the scheduled production date - the date they HOPED to build the car when the order was first entered into the system.

I have a good example here, my 1970 Valiant 4-door was scheduled for April 3, 1970 but the Mylar door sticker shows that it was built in March. So, for some reason it was built ahead of schedule.

By looking at the date codes on the original parts, it must have been made in the last ten days of March, but there is no way to know the exact build date.