The carbo mainsheet fiddle block is made by Harken and comes with the shackle.  It attaches to the end of the boom to allow for a 3:1 purchase on the mainsheet.  This is standard on the daysailing Flying Scots.  Racers who want the option to convert their mainsheet back and forth from 2:1 to 3:1 depending on conditions will also want to use this block on the end of the boom.  The 3:1 mainsheet and the other mainsheet blocks are linked below.