1780 was way late for claiming the invention of a slitting mill. The first slitting mill in the Americas was built around 1640. It still exists, as a historical site and museum.[1] Some of the original slitting rollers for making bar stock are on display.[2] That wasn't the first slitting mill; there were earlier ones in Britain. [1] https://www.nps.gov/sair/index.htm

[2] https://www.nps.gov/sair/planyourvisit/placestogo.htm