They mean copper. Despite you having never head of a copper hammer they do exist.
I think they mean copper.
https://americanhammer.com/copper-hammer/
https://www.amazon.com/Thor-Copper-Hammer-Size-425G/dp/B0001P0Y1Y
https://hammersource.com/copper-hammers/
https://www.mscdirect.com/industrialtools/copper-hammer.html
https://www.gearwrench.com/striking-and-struck/hammers/gearwrench-69-480g-1-1-2-lb-copper-hammer-with-hickory-handle.html
https://www.blacksmithsdepot.com/copper-hammers.html