I've seen some rad supports where the stamp was done very carelessly, on my '73 Challenger for example it was struck on an angle instead of parallel with the support and very nearly ran off the top of it. I wonder if there are any mis-stamps out there where only some of the numbers hit the metal. Did you check the side that faces the radiator? Could it be the stamp was done very faintly and the paint covered it completely? You might want to strip down at least the driver's side of the support to bare metal to be sure, if you haven't done so already. With only two VIN stamps into the metal on an ebody it would seem strange that the factory would let one of them slide by, but I guess anything is possible.