Boltmasters Imperial UNC Slotted nuts are used in applications where vibration or motion might undo the locknut.
Slotted Nuts are often used with a bolt with a pre-drilled hole, where a cotter/split pin can be used to secure the fixing and prevent movement.  For a flusher finish where the pin doesn't protrude outside the A/F of the nut, check out our Castle Nuts.