Genuine hypo-allergenic OD watch cap.  This is the Cap, Mechanic's, A-4.  An OD knitted wool cap without the small bill sewn in and the sides can be turned down over your ears for warmth.  This is a very comfortable cap that can be worn under the helmet in cold weather.

These are "Genuine government issue" "made to Department of Defense specification" according to the tag, and there's no seam down the back.  To tell you the truth, these are not the 100% wool I thought I was ordering.  They're 100% genuine acrylic and hypo-allergenic.  The color and the size aren't bad, though.  Considering that other vendors are selling theirs for $20 these are still a good deal and will be great under a helmet.  They're priced accordingly.