Andy, according to the manual
that came with your KitchenAid Model FGA "Food Grinder Attachment" - the retaining ring is simply called a "ring":
Insert the grind worm (A) into the grinder body (B). Place the knife (C) over the square shank at the exposed end of the grind worm. Place grinding plate (D) over the knife, matching the tabs of the plate with the notches of the grinder body. Place ring (E) on grinder body, turning by hand until secured but not over tightened. Note: Use wrench (F) for removal only, do not use to tighten ring.
If it is less than a year old you might be able to get a new one for free from KA Customer Support (1-800-541-6390) - otherwise I would call the number Uncle Bob posted (1-800-935-0062).