I always wondered if the rings on these were used to try and catch the blade's point, much as the long quillons were used?
Being that its used in conjunction with the rapier, such a design for deflecting and possibly pinning the blade tip.
Thinking through that some more, I guess it wouldn't make sense to allow the point of an opponent's blade to pass through the ring along the defender's hand!! Interesting why they left the ring open on these early pieces, which later became closed guards...