This olive drab rubberized hood is designed for complete protection of the head and neck from chemical agents.  Never issued, sealed in original packaging.  It has an elastic band around the face, so is a universal type fit that should work for virtually any gas mask.  Also has adjustable straps that attach under the arms to keep hood from working its way out of your MOP suit or clothing.  US military issue.  The gas mask shown in the photo is for illustrative purposes only and is not included.