Constant current limiting circuit
This is an interesting solution, not a constant current source. An ordinary 2.7k resistor in a given input voltage range.

It will be much more interesting to watch at lower supply voltages. The characteristic will have a phase with a negative dynamic resistance between 5 V and 14 V. It's a two-terminal device that exhibits an area of differential negative resistance much like a slow and high voltage tunnel diode.

If this is indeed the case, then in this range we can also build an oscillator with it. The oscillator already works with a supply voltage of 6V.

With a supply voltage of 12 V, we are still in the negative range and the oscillator is working.

At 17V, we already leave the negative resistance range and the 2.7k linear positive resistance phase begins.