The SPD on the fender tag is just when the factory "planned" on building the car. Most of the time, the vehicle was NOT built on that day.
Sometime in 72, the door stickers were changed and an MDH code was added to them. My 73 Challenger has an SPD of 927(September 27th) however, the MDH code says 101212.(door sticker says 10-72) The car was built 2 weeks after its SPD.
If you have an earlier 72 or older car without an MDH code on the door sticker, there is no way to know the exact date the car was completed.