The SO (sales order) number was on the paper order blanks for each car, so the SO came first, prior to any model type being specified or options checked off or noted.
When Chrysler received an order form with the SO, they assigned a VIN to that SO / car.
They tracked the order for the car via the SO number, so it was stamped in the body.
Starting with the 1969 model year, the VON (vehicle order number) replaced the SO & they also started stamping the partial VINs in the bodies for ID purposes.
The SO & later VON both share their origins to the original blank form pad pages to fill out to initiate the actual order for a car.