The plywood gussets that cover the top rib of the rudder are made from RS-533 just like the other gussets. This is not mentioned in the plans or manual.
On drawing 9 it shows how to cut the RS-535 for the bottom rib gusset. At first it was not obvious that it depicts cutting two parts from one piece of material.
Don’t forget to cut the slots for the control horn before you glue those on.