Based on VIN sequence, #9193 was built well after the transition from Maggiora Carozzeria to Embo. I have factory documents on record that show #9191, for example, to have been an Embo coach, sold in 1981 (factory invoice dated April 17, 1981). This, by the way, calls into question the owner's claim that the car is a 1979. It is almost certainly a 1981.
As Julian says, a GTS model with a VIN after #9100 is very rare and, being in the middle to the GT5 production run, the car would have been built to a special order (although it could be argued all De Tomaso cars built during this period were, to one degree or another, custom orders).