Musicman Sabre 79 bass pickups and switching

I just got me a Musicman Sabre bass from 1979, and was wondering whether someone had any idea about the pickups and their switching :

1. Are the 2 pickups (slightly smaller in size compared to Stingray) wired in parallel or in series ? There are only 2 wires (black & white) coming from the pickups…

2. What does the phase switch do exactly ? From what I read, it is supposed to put the 2 pickups out of phase. But what I hear on my bass is a different sound when any of the 2 pickups is soloed or both together :
– soloed bridge or neck pickup : the tone with the phase switch on is very interesting, sounding sort of fretless or upright like, with less bass but still usable by slight (quarter of a turn) bass knob boost.
– both bridge & neck pickups : the tone with the phase switch on is really thin and not very usable, even with bass knob boost to the max.
So, since the tone with phase switch on is different in the 3 pickups positions I am really wondering what does this switch do …any ideas of phase switch modifications ?
Here’s what I could see on my 79 Sabre, suspecting the modification is beyond the switching system, perhaps in the preamp ?

Cheers,
Zak