If you've seen pictures from Vietnam, you'll see these hats quite frequently among the mix of other headgear. This was the common "hot weather cap" and this is the second pattern, introduced around 1969/70. These feature a synthetic sweatband, and only have half the stiffener going up the front of the cap.
These are original new old stock from the 1980s, and are great for a late 'Nam impression, or just for everyday wear. Genuine surplus, great price, great hats.