The is the casting that goes between the rudder blade and the tiller arm. If you're replacing this casting, you'll need new rivets to attach the tiller arm (qty 3 of HC 8011231). Since you'll have the rudder apart, you probably should replace the rudder bolt and nylock nut also. Depending on the age of the rudder blade, delrin spacers may be handy to eliminate any slop.
Used on 17SE, 17Sport, 18, 18SX and Miracle 20, along with the older Waves Getaways.