Hi Shaheen!
you still doubt that A-4 and 5 doesnt sport flex nozzles with servo mechanism?Really? Well Thats not a "solid plate" my friend(refering to your picture above). Itz heat skirt and yes it can flex. The nozzle of Agni-4 and 5 can flex. The deflection of the nozzle wont be much but it is enough to pitch and yaw the missile. Btw,designing such a sensitive servo control for nozzle isnt that easy for strategic missile!
PS- Did you know the very first agni-3 missile test back in 2006 failed because of absence of any kind of thermal skirt in missile. This led some of the exhaust gases from itz flex nozzles to enter the motor and fry the control electronics that controlled the nozzle. They fixed this issue by applying a heat skirt to prevent any gases from entering the motor.
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Here is another pic. Full engineering design of the system deployed on smaller prithvi missile-
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Heck even older Prithvis had flex nozzles! I am sure Now all your doubts have been cleared?
PS-Kindly note what I have posted now is already available and published in the form of papers,jounrals,magazines and seminars. I havent even touched the part that has still not come out in open--yes I'm alluding to A-6 and K-5!