This is the turnbuckle that pairs with the standard 3/32 forestay extension (see link below) and is standard on all new Flying Scots.  The forked end attaches to the toggle underneath the bow and the forestay extension threads into the other end.  A safety wire is included to prevent the forestay extension from turning on its own and accidentally unscrewing from the turnbuckle.  We always recommend a minimum of 4-5 threads for safety.