One can date WW1 brodies by the type of steel used which changed during the course of the war. Confusingly you get "magnetic" and "non magnetic" brodies although a magnet will stick to either. Someone did try and explain this to me but it failed to stick (and I studied foundry work at school!) But the point is different steels were used then so there is no reason why something similar may not have occurred in WW2. As collectors of WW1 material can distinguish the two types without having to melt down bits of the helmet there may be a way of detecting any differences.