They are a toweld material and labeled "Extreme Cold Weather" they are essentially thermals and are still issued to the likes of Royal Marines for training is Norway to this day (As far as I know)
You can get the tops used for about £4 or £5 in most army surplus shops or around £10 brand new - The long johns can be had for around £3 - £4 used and about £8 brand new.
These are FANTASTIC, much better than the type you get from most retail shops and cheaper too in a lot of cases. As with anything used by the military they're made to last and work. I have about 4 or 5 tops and 3 sets of long johns and they are absolutely amazing for the money, a must for all BOBs and just to have in your house / vehicle in case of emergency, they do keep you VERY warm and I practically live in them in the winter months.
Just a heads up for a cracking piece of kit


