This is a protective covering for the polycarbonate shield for Norman Thagard'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.
NASA donated the cover when it transferred Thagard's spacesuit to the museum in 1996.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.