That's must be the ugliest schematic i've seen so far. :shock:
Part that is really disturbing is lack of connection points and
the drafter doesn't even know how to draw a straight line.
Anyway, the schematic doesn't show everything. Important part
is not shown and it is before rectifier (center tap from the transformer
is probably not connected to ground but to emitters of the transistors).
The ZN423 is not a zener diode it's voltage reference.
If you want to use 12V zener, it is not connected properly (ends
are swapped around).
T2, D2 and R4 are used to shut the output down when
emiter is biased negative in reference to ground (foldback circuit).
At the same time first transistor would turn on the
LED (probably red) to indicate overcurrent condition.