It could be a repair chain link that's been pulled out of shape. Or, chain used to be hand forged, so it might not be a repair link, it might be a link that wasn't finished, because making a chain it would have to be placed on the last link, then closed and forge welded. I've never made a chain, but if I did, I think I'd make up dozens of links just like that one before I started linking them together and welding the individual link. When forge welding horseshoes the ends are scarfed just like that link, over lapped, heated to a red heat, borax sprinkled onto the part to be welded, put back in the fire and brought up to a welding heat, and tapped together so that the iron molecules actually flow together, and it's welded. Sounds easy, and it is after you've done it a few times.