The current control diagram for the Z3 shows the middle switch being used to toggle between 550/250 Hz for the mid frequency. However I've seen a video review where it says that this is the series/parallel switch for the middle pickup.. On my Z3 (late 2011) it appears to operate in the latter mode as when I select bridge or neck pickup it appears to have no effect..
Can you confirm whether I'm imagining this ?

Cheers
MArk