If it's not already clear, in the table the defective part is highlighted with red, and the replacement 


Existing sensor:Existing Indicator:Defective part replaced with:
Defective: 879 wind sensor. 879.5, 879.521 or WSDI(-1)WSS 550/750 wind sensor + WSI interface box
879 wind sensorDefective: 879.50, 879.521 or WSDI(-1). 

Whole system replaced: WSS 550/750 + XDi-N Wind incl. NX2
WSS+WSIDefective: 879.50, 879.521 or WSDI(-1). 

XDi-N Wind incl. NX2. Directly connected to WSS. (WSI not needed)
WSSDefective: WSDI-2 XDi-N Wind incl. NX2
WSSDefective: WSDI-2+NCI-1XDi-N Wind incl. NX2
Defective: WSS/WSS-L

Any 
WSS 550/75