Dont know why they don't show the washer and nut, slip up in the drawing department maybe. I had this happen to mine. Why they come loose i don't know, Brake vibration .? I did get mine apart without removing the gear i got the split pin out but found it impossible to get back in so i used a self locking nut and plenty of lock tight. had to make a spanner to fit. Hope it stays where it is !!
Dave