This is the liner for the "CVC", or Combat Vehicle Crewman, helmet. Often mislabeled as a tank helmet, since it is used in all tracked vehicles. And some wheeled as well. The liner supports the communications gear normally seen with it. Microphone, speakers, etc.. The complete helmet and design is still in use today, with some changes. Ballistic kevlar shell and changes in the comms pieces such as a flexible mic, BOSE speakers with directional microphones that assist in hearing direct conversation and sound when wearing the helmet outside of the vehicle or disconnected from the comms system. The more modern ballistic shell you will often see used by Special Ops types after being modified to wear as a normal helmet, since it can be worn with comms equipment.
I still have one of mine that I used. Or rather, separate components that complete a sample. Older shell from the 90's, with a more recently used liner with a mix of new and old parts.