I'm assuming the part is proportional in size to the pencil? Thast's part of the snap ring retainer that holds the starter drive in place on the armature shaft. Not terribly important, other than the fact that the end play on the starter won't be correct. As long as you don't remove the starter and dismantle it, you won't miss the piece much, and nothing can fall off because the drive assembly is retained by the starter nose cone. Fix it whenever you've got the starter off next time, and if you're installing a rebuilt starter, the new one will have a complete retainer ring as part of the new assembly.
Jerry
