To better cope with German anti-tank warfare, especially the 88mm gun in June 1941 a set of add-on armour modules was created to fit it to KV-1 Model 1940 tanks in service or to newly manufactured KV-1 Model 1940s,
The add-on armour modules consisted of armor plate of 20/25-30/35 mm thickness and could be mounted to KV -1 turrets (spaced armour) and hulls.
Designed as a stop-gap-measure, severe shortage in supplies and production capacity led to two modification:

• Modification 1 for the turret only and
• Modification 2 for turret plus the upper and lower hull sides.

The new vehicles received the designation KV-1 Model 1941 s ekranami (with add-on armour) to distinguish it from the Model 1940 LKZ..

Source : Tankograd - KV-1 Soviet Heavy Tank of WWII - Early Variants, by Jochen Vollert