JACKETS
M-43 Liner
The M-43 field jacket was designed to be able to take a warm, alpaca/wool pile, liner for when conditions demanded. Alpaca/wool pile was also used for high altitude flight jackets due to it's extremely good insulative qualities. Whilst officially during WWII it was not permitted for GIs to wear the liner on it's own, there is plenty of photographic evidence to show they did.
As a modern day wearer this liner functions very well as a stand alone garment and thus it is very acceptable to wear this just as a jacket in and of itself.