Something not right with the schematic. S06 shows it setting a voltage across C3 (negative?)between ground and the -ve side of the cap. But, that line goes off to the R01 network where you show it going to ground.
I also doubt that transformer pins 1,2 and 5 all go to ground, though it may be possible if each is from a separate secondary winding.
Try to check those traces, If S06 is indeed across C3 like in the diagram, it would be creating (somehow) a negative voltage.
Now, if C6 was also a 47uF like C3, then I would say S06 is a 7.5V zener like S10, and both are used to create negative voltage supply.
This all said, the fact that the part sheets clearly labels Zeners, but not S06 as such, I tend to lean back to assuming it is just a regular small signal diode. This all depends on how accurate that parts list is...