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VR4341 sets the output current of both output current sources via their base
voltages which biases the output transistors so it is a reference application
instead of a clamp or level shift. The value is not critical and will not
affect gain or zero. Current through the zener diode is about 1.8 milliamps so
power dissipation is about 11.2 milliwatts.

I always have 6.2 volt zener diodes on hand as a standard part but grading a
bunch of small signal bipolar transistors for a 6.2 volt reverse breakdown
voltage from 1 to 4 milliamps should work fine here. If necessary the value of
R4344 could be lowered to increase the current through the zener diode for
better regulation.

The low zener current suggests that the original diode would work if R4344 was
lowered in value to raise the operating current.