***DISCLAIMER***
I don't know a thing about flatwounds
With that said, I know a lot about strings and their construction and to my knowledge the only "nickel" flatwounds made are Thomastik-Infeld Jazz Flats and maybe Pyramid Gold Flats. I think that all other string makers use stainless steel. Rotosounds Jazz Flats are monel. Chime in a you flatwound lovers.