This is a protective covering for the polycarbonate shield for Dennis Tito's Sokol KV-2 spacesuit. The purpose of this covering is to protect the helmet shield from scratching while the suit was not in use.
Dennis Tito donated the cover when he donated his spacesuit to the museum in 2002.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.