The Pantera was assembled in two steps. The chassis were built in Turin Italy at the Vignale coachworks. That build date was encoded into the VIN number and stamped on the coach. The coaches were trucked to Modena Italy for final assembly, where the drive train, suspension, brakes and steering were installed. The date of final assembly was attached to the driver's door with a riveted tag. During the Ford production years the "final assembly" date was usually about a month after the date the coach was assembled (the VIN date). During the "Post Ford" years the final assembly could have taken place months or years after the date the coach was assembled.
The Pantera with the highest chassis number manufactured for North America, and imported by Ford, is #7380. This is very well documented. Vignale stopped manufacturing Pantera coaches in August of 1974. We know that De Tomaso acquired about 150 to 200 left-over chassis from Ford/Vignale afterwards. The build-dates of these coaches are coded THPNPU or THPNPM. That's the date encoded in the VIN, not the date on the door. Notice THPNRT is not a 1974 build date, and was never stamped into the coach of any Pantera built at Vignale ... not originally anyway.
So you see, 7580 (7380 + 200) is approximately the highest possible chassis number originally manufactured at Vignale. The highest numbered chassis actually known to me is #7554. So far none of this explains the 8000 series VIN number, nor the "RT" build date of the coach. None of this jibes with the 4/74 build date on the door tag either. Final assembly in April 1974 should have a chassis VIN in the range of THPNNE06900.
Panteras with chassis numbers greater than 7380 were slowly assembled by De Tomaso after August 1974. They ran-out of spare chassis in the early months of 1976. The first non-Vignale chassis, manufactured by Maggiora, was #9001, circa mid-1976.
All Panteras with chassis numbers in the 8000 range have been re-numbered at some time in the past.
Ask Chuck for his experience in the matter.