I agree, I would buy Swiss
Nicholson files were reduced in quality by Our Gov't a coupla decades back. Used to be Nicholson coated their freshly cut files with "cyanide loaf". This was a mix of potassium ferrocyanide (only mildly poisonous to handle, in spite of those 'cyanide' sylables), flour and bone black. It gave the files a light casehardening, although they were only heated for a few minutes, 1440F, in a molten lead bath. This mixture was used to harden the surface of AISI 1035 (a lower carbon steel) used for farrier's rasps.
Our Gov't decided that Nicholson may no longer use K4Fe(CN)6 in their cyanide loaf. Too poisonous. Then they decided we could no longer use Kasinet, which contained the same stuff.
I looked at the ingredients on sea salt from a popular Cincinnati-based grocery store. To prevent caking, they added just a little potassium ferrocyanide.
Personally while I am metallurgically oriented, I would say that how the teeth are cut has a $#*! of a lot to do with file performance. A year or two back I looked at two sorta identical Nicholson files at Woodcraft Supply. The pricey one was made in USA, I presume Alabama. The far more economical choice came from somewhere in South America. Nosey guy, I looked closely at the teeth, saw how they were cut & opted to spend quite a few $$$ more for the Alabama file.
Now I wouldn't even bother going to a US store, just buy Swiss on-line.