Your button is constructed from three "pieces"... an iron back, and a brass front, which are attached to each other by a brass crimp-ring (which is the third "piece"). Combining that type of construction with the fact that your button shows a pictorial scene on its front (a water-wheel millhouse, for grinding grain into flour or cornmeal), suggests it is what button-collectors call a Victorian-era "picture button," manufactured from the mid-1800s through the start of the 1900s. They were made for use on fashionable coats and jackets. Although picture-buttons were more often worn by women than men, in this case, your button might have been on a Miller's coat.