Paint brush hair identification
My guess is that it's badger hair - the grey-white gradations are typical for these kinds of brushes:



They are quite common, and many manufacturers produce them, including Da Vinci, Isabey, Royal &amp; Langnickel, and Raphael.


  Unlike other hair, pure badger hair is thickest at the tips, giving
  these brushes a bushy appearance. They're excellent for glazing
  techniques, and popular with decorative and faux finish painters
  because of their blending ability.  source





  Badger brushes are [..] more springy than bristle, and they have a
  softer touch. They cost more but seem to last forever (I have some
  which are twelve years and counting). source