Fender tag
My point of the engine assembly date (stamped) was that it is most likely more accurate and is typical close to the actual assembly date.
On earlier cars there is no way to know what the actual date is so you have to play detective and make a close approximation, I'm sure on most cars will find all the date codes stamped and otherwise will be in a range that the schedule production date looks good enough. I haven't done research on this but would guess that most cars are built somewhat close to the schedule production date.
As long as I have had my Barracuda (1985) I have understood the schedule production date to be a fluffy number, I see no way a car could go down the assembly line in Michigan and in one week be at the dealer in California in 1966. Even knowing it most likely was built earlier I still use that date as it is all I have to go on.
Alan