Here is a Freehand Zero modified with weight rings. The rings are from the old online store Infinite Illusions or yoyoguy.com. I'm unsure if the rings were sold separately, or came installed in the yo-yo.
There is another Freehand Zero in the museum collection with this kind of weight rings, except that one has rings that are engraved "Duncan". So it's likely the rings were made and produced by Duncan. The other example is the Enyo Rings Freehand Zero exhibit.