According to my (limited) resources, a hexagon nut with slots across one end is more often called 'slotted', while if the slotted end is also reduced in diameter and round, it is 'castellated'. The two terms are often used less carefully, though. Both styles tend to have more depth than a modern plain nut. Because they are a bit on the small side for split pins, adjacent nuts are often wired together instead.
Cheers, Tim