It has to be based on the 1300 GT, because the 1700 GT wasn't launched yet when this Viersitzer appeared. The hood scoop is shorter than the one that came with the 1700 GT (and was also used on the 1300 GT from that time). This Viersitzer was an attempt by Frua to sell another design to Glas, but he wasn't successful. The Isaria name was used by Glas for his agricultural devices. I don't think that it ever was used for his cars.