This is a new US Army M-65 Trouser Liner Medium Regular\Short nsn 8415-01-180-0373.

The trouser liners are quilted and made of polyester batting covered with 3 plies of ripstop nylon cloth. The trouser liner has buttoned side openings for easy removal.The trouser liners have been redesigned to make these items capable of being worn independently of their respective outer garment.