The extended vertical fin fairing is Airbus’s response to the design department’s decision to remove the vertical stabilisers on the T3 variant. The first T3s, without the fin extension, had (still have) an uncomfortable tendency to fishtail at high speed, especially in turbulent conditions. The fin extension is now offered as an optional refit for those early airframes. Makes you wonder how come the test pilots signed it off in the first place.
The fin replacement was free, but Airbus then schwacks you 30-40k for the autopilot software mod to go with it.