Castle Nut for Brake Drum, 36mm, without shoulder
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 reproductions
36mm head M24x1.5 without shoulder