I would say replacement. I don't think that number could be ground off without ample evidence.  Apparently there are two more places on the fender aprons that have numbers on them.  The problem is that you have to remove the fenders to see them.  They are located, one on each side, on top of the apron flange, just behind the shock towers. 

If the current left apron is a replacement, it is a separate panel from that part behind the shock tower so there should still be a number in that secondary location.

Did the door tag look like a 50 year old tag?  Still does not matter much, the whole door could have come off a different car.

Last edited by boomyal (5/07/2017 1:02 PM)

What number does the title have on it?

My 1965 has the same vin # on the drivers door tag.....I was told it was the warranty # but, it matches the vin on the fender.....As was mentioned, there is the " secret," place on the passenger side under the fender
lip. It is put there in case of a theft to verify the correct #..My Mustang has been registered
to me , originally from Calif. In 1969...The warranty ,# ...vin # and registration papers are all the same. I knew the original owner who purchased it in Oct. 1964 in San Jose , Cal

You can try looking from underneath the fender aprons at the other two locations. Sometimes you can read the numbers from below if they were heavily stamped.