The carbo mainsheet becket block is made by Harken and comes with the Ronstan bow shackle for attaching it to the rudder head strap.  It is important to note that the newer Flying Scots no longer use the bow shackle but instead have a stainless ring that the becket block attaches to.  See link below. On the racing Flying Scots this is the block that is on the end of the boom for the 2:1 purchase while on the daysailing Flying Scots it attaches to the rudder head to allow for the 3:1 purchase.