What is you "VIN" code? Is this an engine or cab/frame code?

Maybe not a change in the Sedan Delivery, but there was a change in the Panel truck between 1946 & 1947.

The break in the trucks took place during 1947. I do not know when/if the break in Sedan Delivery models.

GM did not use the terms 1st and 2nd series for the 1946 trucks sold in 1947, and the new Advance-Design 1947 trucks.