1942 10 the British army successfully developed a bulletproof vest consisting of three high-manganese steel plates for the first time. 1943, as many as 23 kinds of body armor were trial-produced and officially adopted in the United States. The bulletproof vests in this period were mainly made of special steel. 1In June, 945, the US military successfully developed a bulletproof vest made of aluminum alloy and high-strength nylon, model M 12 infantry bulletproof vest. Among them, nylon 66 (polyamide 66 fiber) is a synthetic fiber which was invented shortly at that time. Its breaking strength (gf/d: g/denier) is 5.9 ~ 9.5, its initial modulus (gf/d) is 2 1 ~ 58, and its specific gravity is 1. 14 g/(cm.

That army used German weapons. It's probably steel plate inside.