Question on the Challenger 2
 

Some versions of the C2 have silver/light-grey rectangles on the sides of their turrets that look like shutters, and they have two smaller "shutters" on the backside of the turret, to.


http://www.enemyforces.net/tanks/challenger2_2.jpg

I wonder what these are? A greeting to the enemy: "Knock here and enter"?

They also have two apparent "marking boxes" on the frontal side of the turret, like the Abrams M1 also has. Often these are even saved from any multi-coloured camouflage patterns the rest of the hull is painted with, and thus become visually even more noticable. What are these markings' purpose? Plates of addon armour to be placed there? Never have seen that addon armour on the M1 or C2.

These are TIS identification panels.

They have a contrasting rate of heating&cooling, and a resulting difference in temperature and visibility, either during the morning 'warming cycle' or during the evening 'cooling cycle'.

The idea is that it makes IFF easier, while remaining purely passive.

This is similar to the large 'V' painted onto the skirts and used for IFF on the daysights.

Ah! Every day you learn something new!:DL