I did buy Kolinsky Sable brush from Winsor & Newton, Series 7 size 000. It turned out to be a disappointment. No different from synthetic brushes, and it cost ten times more. Some say they have quality issues, others say there are counterfeits sold on Amazon:
Brand new Windsor & Newton series 7, anything to do to fix it?
byu/beardmire inminipainting
Then I got Raphael 8404 size 0. This is what I was promised from Kolinsky Sable brushes. It didn’t make me an amazing painter, but now I’m able to paint warts on my rats.
Seven rats done, 13 rats to go.