IS tanks didn't have the same invincibility as the KV enjoyed in 1941, but their armour was still formidable against German guns.
CAMD RF 3153-1-6
The handwritten text reads "This IS tank has 16 dents from enemy artillery shells. The commander of the tank, Guards Junior Lieutenant Neelov, destroyed 3 enemy tanks, 2 enemy SPGs, and 7 enemy guns."
The printed text reads "In the aforementioned battles, our IS tanks played an active role. Brave tankers fearlessly fought enemy tanks, and destroyed enemy positions with guns and tracks."