Mysterious? I thought this is a solved problem. Especially also taking into account the boarders of the Roman Empire and the places where they were found. https://www.youtube.com/watch?v=76AvV601yJ0
These were expensive and difficult to cast. They are made of metal which is very expensive at the time. There's no reason knitting would require a complex metal cast object of this kind when a wooden jig would be far easier and cheaper to produce.