Slotted Nuts - UNF
Often called an axle nut as this is where they are most commonly found, a slotted nut is a hexagon nut with turrets and slots on the top which resemble a castle wall. Slotted hex nuts are used in conjunction with bolts that have holes in the end through which a locking pin can pass to lock the nut into position and prevent any further rotation. The locking pin is usually a split cotter pin, an R clip, a spring pin or a safety wire.
The diameter of the castellated section is the same as the main wrenching section of the nut (unlike a castle nut where the diameter of the castellated section is slightly smaller than the main wrenching part).
About UNF Thread
UNF stands for Unified National Fine. It is a common imperial thread type with more threads per axial distance than its UNC equivalent.
Unified thread types (UNF and UNC) were introduced at the beginning of World War 2 to enable the interchangeability of machinery manufacturing between the USA, Canada and Great Britain. Still commonly found on a vast array of old machines and vehicles in the UK, UNF and UNC threads continue to be the standard thread in the USA and Canada.