The VIN-derivative as it is known was stamped by Chevrolet soon after receipt of the completed body-it was done after paint. We have seen errors. I personally looked at a '69 last fall in IL that had no hidden VINs and a mis-stamped VIN tag.
I wonder why they would stamp after paint...wouldn't that leave exposed metal and start rust? Apparently it didn't cause too much rust, at least on dry climate cars. It seems like a bad idea to send exposed metal out into the "field".