I've seen corrected (or double stamped) apron vins, but never on the data warranty tags. As stated above, they were reverse stamped. I'm guessing someone changed the car's color sometime in it's life, and decided to change the data plate to match. No need really, as the tag is not for registration purposes; it clearly states that right on the tag itself. I wouldn't be worried about it- doesn't effect the car's value imo.
Working on old cars teaches us patience... and every curse word imaginable!