These are a 3.5mm Dropline or "Bran" Swivel used for making droplines and deep sea rigs when fishing for Blue Eye Trevalla, Gemfish, Striped Trumpeter and Hapuka.
They can even be used for setting up teasers and spreader bars for trolling for Tuna and Billfish.
They are suitable for mono up to 3.5mm or 1000lb, have a 4/0 swivel are easy to use and are sold in a pack of 10.