I think its just Copper coated with Tin.
I just put mine into a seperate sack & keep it with the copper & then mention the 'tinned' copper & put them together.
They know what it is, its Copper.

'Tinned' is probably a misnomer, its Tin, but solder coated copper (wire or a soldering bolt) is called "Tinned" even though its a mixture of tin & lead (solder).



I find it on braided copper wire (TV signal cable & such) & odd, mostly old wire.