Yeah, flattening both ends, overlapping them, punching, then riveting with a wedge, flattened and cut from the same wire stock used for the rings. I don't have a proper macro lens for adequate detail.
Any place I could buy such pliers, or at least detailed specs, so they could be fabricated? I guess you'd have something like the jaws functioning as a top and bottom swage to get consistent results on the flattened ends, maybe a protrusion that does the punching, all in one go, then a second set where you'd have mirrored concave bits to compress and dome the rivets?
I'll have to bother some people there, when I can get around to it. That looks like a pretty good source for some other stuff either way.