The SON (Shipping Order Number) is what triggered the building of the car. It related to the original order of the car and during the 1960's, at least before 1968, was the number they used to track the vehicle from receiving the order to shipping the car from the plant after the car was built and rolled off the assembly line. Which is why the SON (Shipping Order Number) appears on the body tag as well as the car and the broadcast sheets printed for various parts and procedures as the car rolled down the line.
The VIN was not assigned until the car was being built. If you check the 1960-1965 VIN plates the last eight digits (model year, assembly plant and sequential production number) are pre-stamped - the numbers are all raised. The car line and series were stamped onto the plate from the front before it was attached to the body..