Legendary aircraft to quit flying soon

Bob wrote:

Excellent photos of the plane. Brings back memories of making plastic
models as a kid, I'm sure one was the Viggen.

I loved the Viggen when I was a kid - I still remember making and painting
Viggen models. At the time I was very curious about the 'circular hole'
around the engine tail cone - I never figured it out. Can anyone help me
understand what it did, and if it worked, why other aircraft do not have it.

Thanks,

Hilton