This is an old socket corner chisel. A large tool it would have been used to clean out the corners of mortices in timberframe work. Someone many years ago has used it as a sort of turnbuckle or tourniquet to tighten this section of fencing. I suspect that it may have been used in the framing of the old barn on the opposite corner of this field. I first noticed it over fifty years ago while walking the hedgerow behind my family home. I have often considered freeing it from the wire but I guess it just belongs where it is.