I remember reading about how tank crewmen often used football helmets as their protective headgear back in the day. As a former tank crewman myself, I can tell you that the CVC ones used today really aren't much different than the football helmets used those many years ago (the helmets are meant to keep your head from hitting objects inside the tank, not to protect from gunfire). Anyhow, this auction popped up on Ebay and appears to be one of those helmets:

http://www.ebay.com/itm/VINTAGE-RAWL...item3ccd07b490

It still retains the cord that plugged in to the communication system, which was used to either transmit to other vehicles, or the intercom system to speak with other crewmen.

Very cool item.