Rear axle/ brake drum castle nut.
The 36 mm nut that holds the rear drums onto the rear axle, has a castellated head for the split pin.
Castle nuts are tightened to a high degree when fitting so the thread need to be in good condition.
If the head of your nuts are worn or threads are in poor condition replace them with these top quality items.
36mm head M24x1.5 with shoulder