They're not listed because they aren't "parts" but rather a part of the saddle. They are pressed into the underside of the saddle to prevent the block from falling through if you loosen the locking bolt too far. They also prevent you from putting the string end in too far. Without them, you can put the string in too far and then the string end hits the baseplate when you're fine tuning. It's no big deal. You could try to press them back in but they will likely pop free again as you poke the string down in there during a future string change. You could use superglue to hold them in if you want.