This Fin shaft ring is used on shafts with sharkfins to convert a free shaft (untethered to gun) into a lineshaft (tethered to gun).
The ring is stored on the end of a wrap or 2 of mono until it is needed and then slid over the end of the shaft and flopper. After the shaft is fired from the speargun the ring slides down the shaft where it catches on the sharkfins.
This ring is an improvement over any other type ring as the hole is designed to fit over any size flopper. It also has a unique dimpled shape for better handling with gloves and to flow through the water easier.
Made from a high quality stainless steel, and the holes are countersunk and polished so they will not get stuck on shark fins or damage mono.
These are easily the best you will find anywhere.