The Wool Knit Cap was designed as a second layer to be worn under the M-1 Helmet. It was to help have the helmet fit more snug and comfortably as well as provide warmth. However, the hat, more commonly and fondly known as the "Jeep Cap," took a life and purpose on its own as GIs started wearing it without the helmet when not under direct fire... much to the distatisfaction of by-the-book officers.
The hat was made publicly popular by the hit TV series, M.A.S.H. Though the show took place in the Korean War, the hat that "Radar" O'Reilly made famous was the WW2 pattern and is the same pattern that can be found in surplus shops today in a variety of colors.
When looking for a passable jeep cap, be sure that the material it's made with doesn't look too synthetic and that the color is more of an olive drab color as opposed to the dark greens that are more common in surplus stores. The hat should fit snugly around the head. It shouldn't have too much material extending from the point where the bill meets the cap and it should have a 6-star stitch pattern in the crown.
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