On my 69 GTS the radiator support was never stamped. They just missed it all together.
That happened from time to time. They also almost missed on the trunk stamp. Probably a rough Monday morning for the stamping guy.
I have seen screwed up body stamps on LOTS of A Bodies. If I remember correctly my 71 340 Demon has one of the "1's" missing on the radiator stamp. It is 5 hours away at my in-laws ranch in storage right now otherwise I would go take a picture for you.
Keep looking on the rear deck / Package tray. Also the trunk lip. All of my 69's are stamped on the drivers side of the trunk lip under the seal. BUT.... None of them are in the exact same spot. It is like they had a stamp, and the car was going buy at 20mph and the just hit it where they could LOL.
Up to 68 models they did not stamp the VIN on the body stamps, it was the order number and should match the fender tag. From 69 on it was the VIN that was stamped on the body stamps.
We get e-mails at the GTS registry from folks all the time asking about their VIN stamps on their cars. Most of the time it is a 68 owner that thinks they have been cheated because the body stamp doesn't match the VIN. Boy are they relieved when we have them look at the order number on the fender tag LOL.
It will be several weeks or more before I head back to my in-laws place. I will check on the Demon, and on my 70 Duster to see where they are stamped. If I can remember LOL.
-Dave