Are you speaking of the European-only or U.S-only GTS? The GTS was DeTomaso's standard narrow-body model sold through 1987 in Europe. The U.S-only GTS was sold in late '73- early'74 in limited numbers and had 'GT' in the serial number, while Euro-GTS did not have it in their serials.
With exports going in both directions across the pond the last 15 years, the two models have gotten mixed. For instance, '87 Euro-GTS #9483 is in New Jersey, owned by GT-40 racer Charlie Seabrook.