Dachstein wool mittens are boiled over a wooden form to shrink them to an exact size. They are very dense and VERY warm.
I wore a pair under leather mitten shells while patrolling the Nordic events at the 1979 Pre-Olympics at Lake Placid. At -15 F. at the Biathalon venue they were warm, maybe a bit too warm. At -40 F. (yes) on the last day I had Army wool glove liners under them and had to remove the glove liners as the combo was TOO warm.
So yes,Dachstein mittens are the real deal. I still have mine and sometines use them in below zero temps inside OR GTX mitten shells.
BTW, The only thing that may be warmer is a pair of really thick, double layer fleece mittens I have. I had to buy an XL size OR mitten shell to get them to fit inside.
The company that made the double layer fleece liners only sold them for one year.