Knit Cap

The olive drab "Cap, Wool, Knit, M1941" was standardized in early 1942, intended for wear under the M-1 Steel Helmet. Although the helmet liner had a webbing suspension system, soldiers liked having the extra padding and warmth of the wool cap.

You can find the  original my favorite store  Kaufman's Army Navy   in OD s well as other versions year round .
Kaufman's Army Navy
319 W 42nd St
New York, NY 10036-6911
(212) 757-5670

J. Crew makes a cashmere one for $48.00