Lorenzo, the whole subject is unclear. During 1971-74 (the Ford era) DeTomaso in Italy had contract rights to sell Panteras to the whole world EXCEPT the U.S.A. which was a Ford exclusive. DeTomaso chose to call his cars sold in Europe and the far east as "Pantera GTS". So these are 'std' narrow body Panteras for Europe & elsewhere and were made until the early '90s (unknown total number).
There was a rare (36 made) European variant called the Pantera 'GTS/Gr-3' that was a homologated FIA 'racecar for the street'. The GTS/Gr-3 Registry, maintained in Sweden, shows your car is not one of those 25-or-so factory-built machines. There is a second list showing eleven (11) more std GTS the factory converted some time after the cars were sold & titled. Your car is not on that list. either.
But a GTS/Gr-3 could also be built later in its life by a specialist shop or at home, since the Gr-3 parts were available over the counter at DeTomaso Automobili in Modena, IT or via his concessionaires in many countries. It sort of sounds like your car was of this type. There are a number of incomplete conversions known, as they made hotter street cars than a std Pantera GTS but were still street legal in most countries. Browse to <www.scuderiadetomaso.se> for much more info on the GTS/Gr-3.
Finally, as Jules mentioned, there was a U.S-only variant also called the 'Pantera GTS' (138 made in 1973-74). What defines this one is, all had factory serial numbers beginning with THGTS-xxxxx stamped into the usual places. They were essentially a cosmetic pkg added to a std U.S Pantera by Ford, probably to stimulate sales, using some items developed by DeTomaso for his Euro-GTS version. All rather confusing but standard procedure for limited production car building in Italy at the time.