The official standard for describes Hexagon Castle Nuts with the following specifications:
ISO 7035 technically refers to Hexagon slotted and castle nuts — Product grades A and B iso 7035 pdf
The nut features vertical slots. After tightening, a hole is drilled through the mating bolt or shaft, and a cotter pin is inserted through both the slots and the hole to prevent rotation. The official standard for describes Hexagon Castle Nuts