On some Hawker Hunters, there is a zig-zag on the leading edge of the wing, as shown below.
Why do only some Hawker Hunters have this feature, and what is it for?
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As already answered, the zigzag is a dogtooth extension or leading-edge extension on the outboard portion of the wing. It forces the airflow over the wing into a vortex, preventing spanwise flow outward along the wing at high angles of attack. A lot of modern designs incorporate them, including the horizontal tail of the F-15 Eagle, because they often have a side benefit of increasing airflow over the ailerons and increasing control.
To answer the second part of your question (why do only some Hawker Hunters have this feature?): the dogtooth extension wasn't incorporated into the design until the F.6 variant. Wikipedia's list of Hawker Hunter variants mentions it in a couple of entries.
Basically, the earlier models didn't have it; at some point the designers and aerodynamicists working on the Hunter tested it out. Once they figured out that it was an improvement, they would have made the necessary changes to the production line to make it standard on all late-model aircraft built.
After doing a little more research I found the answer.
Quote from wikipedia:
A dogtooth is a small, sharp zig-zag break in the leading edge of a wing. It is usually used on a swept wing, but also on straight wings ("Drooped Leading Edge" arrangement), to generate a vortex flow field to prevent separated flow from progressing outboard at high angle of attack. The effect is the same as a wing fence.
Where the dogtooth is added as an afterthought, as for example with the Hawker Hunter and some variants of the Quest Kodiak, the dogtooth is created by adding an extension to the outer section only of the leading edge.
Cirrus incorporates this design into their wings as well. Quoting from their site:
The outboard section of the Cirrus wing flies with a lower angle of attack than the inboard section. When the inboard section, which produces much of the lift, stalls the outboard section, where the ailerons are, is still flying. The result is that a stalled Cirrus airplane can be controlled intuitively using aileron.
As an added piece of information, this feature may have something to do with the fact that the Hawker Hunter, for many years, was the only swept wing fighter that could be reliably recovered from an inverted spin.
For this reason the Empire Test Pilot School at Boscombe Down used one for advanced training purposes (and may still do so).