M1 is turned on by R1 charging its gate towards positive rail.
R2+R3 and D2 implement current limiting - if current reaches 15~20mA, there is almost 10V loss across R2+R3 and D2 discharges M1 gate to a voltage barely above M1 source, limiting further current flow through M1.
D1 limits M1 gate to 43V above the negative rail, so for M1 to conduct at all, about 40V above the negative rail must be at its source. Since the negative rail is regulated by zener D3 to -13, that makes about 27V at the output of M1 if there is no load current across R3.
The SCR appears to be a crowbar which shorts M1 output to ground to shutdown downstream circuitry.
Not sure what's the point of the choke.