Definitely a draft/draught (heavy hauling) animal (horse, oxen, etc.) harness buckle. Although it looks similar to a headstall buckle (see the photo below), it is MUCH larger than a headstall/bridle buckle. Compare its size with the digger's hand. Also, there is another difference from the headstall buckle... note that it has an integral "tongue" (prong) on the fourth side, partly obscured by the digger's fingers. It is actually for where four leather harness-straps connect. At this very large size, it might be a harness backstrap buckle.