I also thought that, also if they're resilient mounts for the motor mount to the wood, but you'd expect 4. I also wondered if they were inserts for A or P brackets, but rubber on a fast rotating shaft no good. Also, post a pic of the rudder arrangement. They might be grommets for that. Otherwise I might put them in that spares box, sneeze and lose them, or throw them over my shoulder and move on. Now't like modelling to find out you disregarded that critical part that should have been fitted in a part you can't get to. It's part of the fun, LoL. We've all been there.
Rich