Has the truck been "restored" previously? It's possible as Otto mentioned that the doors might have been changed, although there would be no real advantage for anyone to change to '51 doors, as they're one-year only design and hard to come by. It's possible the data plate was changed. But if the title says it's a 53, then that's what it is per the registration authorities.
Here's some good info to go by.
AD Spotting Guide
