This would have been an antique iron known as a slug iron or box iron. This tool was widely used in the 18th and 19th centuries before the advent of modern electric irons.

A slug iron consists of a hollow metal body with a hinged lid, and it was designed to hold a hot slug (a solid piece of iron) that would be heated separately in a fire or on a stove. Once the slug was sufficiently heated, it was placed inside the hollow iron to maintain the heat needed for pressing clothes.