The car is definitely an L model body, manufactured at the Vignale plant, and appears to be a car originally intended for sale in North America. Its not likely the single slot Campys are original to the car, but stranger things have happened. The body service number and trans number would indicate the car originally had a chassis number of about 6300. I like the color by the way, Panteras look sharp in orange.
I've always been under the impression only the factory renumbered the chassis. That would mean the car has been to Italy sometime in the past. Or perhaps as you mentioned, the car was renumbered before it was sold new ... and then exported to the US as a custom order. That seems like the most likely case.
Knowing the original VIN may put you on the path of a previous owner ... or it may simply prove this was a left-over coach in which final assembly took place in the "Post Ford Era".
I don't know how hard it would be to remove the foot well plate with the VIN stamped into it, but you know the original number will be below that plate. I wonder if it would be possible to read the original VIN from inside the car, looking up at top of the foot well from inside with an inspection light & a mirror?
Santiago De Tomaso may be your only source for unravelling the cars history.