I can't answer your question, but yes, there seems to be two types of sway bars for the Z4. There is no "is" sway bar though. It is rather an "M sport"-sway bar which is used with everything from the 18i to the 35is. Maybe because they are lowered 10mm?
The part numbers are the same for 35i and 35is if the 35i is equipped with the Adaptive M suspension or the non-adaptive M sports suspension.
http://www.realoem.com/bmw/showparts...96&hg=31&fg=10
Same story for Z4 18i.
http://www.realoem.com/bmw/showparts...96&hg=31&fg=10
Apparently the part is used across SUVs, 1-series cars and so on:
Here is the Z4 M sport version:
http://www.realoem.com/bmw/partxref....=on&showeur=on
And here is the Z4 non-M sport version:
http://www.realoem.com/bmw/partxref....=on&showeur=on
Same story with the sway bar used on the 335is, just fyi:
The one used on E9x 335is non-M sport (used with everything from 316i sedan to 335is coupe)
http://www.realoem.com/bmw/partxref....=on&showeur=on
The one used on 335is M sport (also used with everything from 316i sedan to 335is coupe)
http://www.realoem.com/bmw/partxref....=on&showeur=on