I can think of sites where corrals and horse and tack things went on, where iron square and rectangular stuff like that is so common, we just chuck it to the side. Or drop it off at the first trash can we come to. Unfortunately, it's one of those items can cover 200 yrs. Could be as early as the 1750s (I suppose), and/or could be as recent as the 1950s (I suppose). Contrast to a copper or brass buckle, and the color and feel (alloys and impurities and machined "feel") will indeed give you an idea as to the age. But iron just rusts up, leaving the finder little to go on, unfortunately.