Red_Micro and the group,
That configuration is this:

The MOSFET is in series with the MOSFET inside the IC U2. IT is called the Cascode configuration.
The 500V Zener string limits the voltage on U2
The 12V Zener limits the Vgs on the MOSFET.
It is being done here to raise the maximum operating voltage of U2.
Regards,
Jay_Diddy_B