I was under the impression actual stamped chassis # didn't come into use until the second series Task Force trucks.
AFAIK AD first series trucks simply had the alloy plate in the LH door jamb.
Even if the screen print is worn off/weathered and assuming it's still there,you should be able to read the serial number along the bottom,that's how I found mine.
I also thought '51s were a one year only deal,that's unless the doors on your truck have been changed out in the past Raoul.
They for me were the main identifying part for a '51. i.e. turn down handles and 2 piece glass.
