What is the bulge under the engine of the Mirage III and some of its variants? 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?
For the Mirage III, it's a fuel tank (that also allows a camera attachment). See cutaway below (item 157).

Also from Wikipedia:


  The Mirage IIIRS could also carry a centerline pod for conducting photo reconnaissance missions, as well as an integral fuel tank underneath the aft belly; this tank could carry a smaller fuel load, but also allowed for a rear-facing film camera to also be added. When fitted with the reconnaissance pod, supersonic performance was severely diminished.



Source: aviadejavu.ru (composite of two pages: 16-1.jpg and 16-2.jpg; use links for full-res)