What is stainless steel piano wire?

There is no such thing - this is a misunderstanding regarding spring wire.

It is a misunderstanding that piano wire can be stainless steel - this is due to the fact that piano wire is often understood to be spring wire, of which there are multiple versions.

- Piano Wire is untreated spring Wire that offers no resistance against corrosion.

- Stainless steel spring wire is available in several versions, for example, 302, 304 and 316, but it has nothing to do with piano wire in this sense.