The mainsheet extender is made of 5/32 wire with loops nico pressed on each end and plastic tubing covering it.  It does not include a shackle.  The extender is used between the rudder head strap and the mainsheet block and is often used in conjunction with the mainsheet retractor kit or the Palm which are both systems to pull the mainsheet purchase forward and prevent it from hanging up on the aft corner of the boat.