This is the roll pin that the rudder cam pivots on. Rudder castings came standard from the factory with this pin. But removal requires drilling, so most people replace the pin with sister screws when they replace the rudder cam. sister screw are reusable and install with a common screw driver.
Installation of these pins require a special tool and most sailors don't have access to one. Check the Recommend Products below for the Sister Screw part number.