Yeah its the 23mm mount.
Before you order though let me shed some light on my current setup:
1. I bought the homelink auto dimming rear view mirror separately as I do NOT have the touring edition 370z. So the stem may possibly be a different size than yours.
2. The 23mm Mount on mine is slightly larger than my stem, so I routed the homelink cabling of the mirror through the Mount to keep it flush and from moving around while driving.
3. From comparing my base model 370z OEM mirror and my new auto dimming mirror, the stem on the OEM base model seems to be slightly larger than my new auto dimming mirror so the "slack" may not be present in your case (if you have the base model OEM mirror) and sit flush.
Hope this helps
