Small Scale Military Headquarters

Since I have so many, I trying to figure out how to use the tank crewman helmets from my 21C Vietnam figures. What years were these used through? Were there any other countries which used these? And is there a better way to describe them to find more info (best search I've found is a CVC helmet, although most info I find is relative to the modern crew helmet)

Thanks for the help.

IIRC we started getting rid of the VN era CVC in the mid 70's. They were being replaced as they wore out, rather then according so some schedule.