While attempting to create a Kfir in 3D modelling software, I noticed a bulge under the engine on the reference picture (a 2D side view) I was using, which I initially assumed to be a fin. After looking at some 3D images of it, however, it is far too wide to be a fin:
There is a similar bulge in the same position on the Mirage III:
The Atlas Cheetah and Mirage 5 both have a bulge in that position as well, although every non-Mirage-III-based Mirage seems to lack it. The Mirage 2000 has a bulge in a similar location; however, it appears to be much smaller:
My initial thoughts were either ECM or some kind of fuel tank. Does anyone else know what it might be?