This is a large end 3/8″ x 4″ S-hook.  this S-hook works with our C100 – C110 – C150 chain only.   Our commercial quality large end S-hooks are designed to fasten chain to swing hangers and swing seats to help prevent binding. They are plated to resist rust. S-hooks that are not bought from a playground manufacturer do not have the quality control and have been known to break with the first “swinger”.  Tensile strength is 1177 lbs.