The steel breastplate SN-42 is made of 2 mm steel 36SGN, tolerances are 1.8 — 2.2 mm, weight of the bib is 3.3 — 3.5 kg. the protection Area is 0.2 sq. m. was Used in WW2. Was created several models: SN-38, SN-39, SN-40, SN-40A, SN-42, where the number indicates the year of development. They were equipped with mostly engineering assault brigade. The steel breastplate withstand getting pistol and machine-gun bullets, and small pieces. There are cases falling 9-mm pistol bullets at point blank range without damage for a fighter. Steel due to its smoothness and strength could sometimes reject 7.92 mm rifle bullets and blows from a bayonet, but only in tangential contact at a large angle. 
By modern standards roughly corresponds to the bulletproof vest class 2 - as for the weight and protective qualities. This "armor" soldiers usually wore the jacket with a torn sleeve that serves as additional shock absorber, despite the fact that the bib on the inner side there was a special lining. Had a good performance in urban combat.