Hello!
While I'm sure Rob can tell you much more about it, the armored bulge houses the optics of the stereoscopic rangefinder. It basically compares pictures from two scopes (hence two bulges) and using the angle between almost parallel scopes that show the pictures being very close to one another it calculates the distance from the target, which you need to accurately aim the main gun.
https://en.wikipedia.org/wiki/Coincidence_rangefinder
Those bulges should have a narrow, vertical slot in their fronts, that's where the scopes sit. Good luck with your build and have a nice day
Paweł