When in the early 70's the Porsche engineers needed more downforce on the
powerful RS 2.7 to fight the high speed oversteer they succedeed by constructing a new type of rear spoiler. The sales department was not very happy with their efforts and blamed the engineers of giving the RS a "duck's tail".
The ducktail became an icon like the Fuchs wheel or the Turbo. Porsche kept us waiting almost 40 years until it was offered again on the limited edition 911 SportClassic. And nowadays, for $5000, or just half of what a RS 2.7 cost in 1974, Porsche will fit a ducktail on your 991.