In December 1941 the KV series of tanks received a new turret type incorporating selective 110-120mm armour around certain weak areas of the earlier cast turret.
The overall weight, however, remained identical.
Designated KV Model 1942 with reinforced cast turret, the new type went into production alongside the KV Model 1941 with cast turret from Januar to August 1942.
The new turret is externally nearly identical to the previous design, but has wider armour around the turret ring, small changes in the cast structure and an armoured collar around the rear turret machine-gun ball-mount.
The late production variant of the Model 1942 received the new type of hull with the angled rear while the early production batches still got the rounded one.
Also, the new hull still mounted the engine access hatch with the the dome shaped bulge, the later production batch mountet a flat engine access hatch.

Source of Text and Photos : J.Vollert, "KV-1 - Soviet Heavy Tank of WWII - Late Variants", Tankograd Publishing