GSh-6M was a successor of the original GSh-6 helmet and was the first GSh-6 version produced in large numbers (less then 3,000 GSh-6 helmets were produced). Letter "M" most likely stands for "improved". That said, early GSh-6M helmets look identical to GSh-6 helmets and later helmets differ from it only by a redesigned backing plate for lowering outer visor.   Front view of the late production GSh-6M helmet. Note that oxygen hose is located on the right side. My helmet had seen better days and has some bumps on the shell and scratches on the main visor. Forward-right view on the same GSh-6M helmet. Note the pressure valve and visor heating assembly. Left view of the helmet. Note the redesigned backing plate used to lower outer visor. Close up view of a small knob that controls position of the internal visor - shown here in a full up position.