M16 thread diameter, 2mm pitch, DIN 935 bolts.
A hex nut with opposed slots cut into the top of the nut through the centers of the flats. The slots are on the end opposite the bearing surface.
The slots are for the insertion of a cotter pin to secure the nut when used with a drilled shank fastener.
Commonly used in the industry to secure front wheel bearings / wheel hubs to spindles on and trucks.