When you start grinding, if it smells sweet, it's polyester. Don't think epoxy would have broken like that (like what is swelling?), nor gotten so bulbous looking. If it is epoxy, it'll be very interesting to see what it is hiding.
338's Factory shoe was very corroded on one side - mostly because the DFO's rudder post was made with highly alloyed s.s. propshaft - that's what I think anyway. Really hard and showed zero corrosion.
Rudder shoe should be made of nearly inert silicon bronze, along with the rudder post. But it's possible other copper alloys were used.