WW2 HBT yardbird boonie hat. Yet another rare WW2 surplus find. These are WW2 Herring Bone Twill (HBT) sun hats. Constructed of an all cotton, heavy olive drab material. These are unissued with manufacture and date tag still crisp and intact. They are small, size 6 5/8, very very small, but would be a great addition to any WW2 collection.