Ladder chain commonly used on player piano to connect roll motor and transmission. About 4 links per inch (more accurate is 42 links per foot), the most common size I’ve encountered (there is another size, 5 links per inch I encounter on some Baldwins and a few others).
Sold by the foot, orders of more than 1 foot will be shipped as one continuous length.
The chain is steel that has been copper plated.
I’ve restored player pianos for 30+ years.  If you have questions about this product before or after your purchase let me know, I’m glad to help.  I include my contact information with each package and you can also reach me through eBay.