Center Lock Nut Theory
Those are stakes that slightly deform the threads inside the nut.... This creates drag so the nut won't vibrate loose, it won't 'rattle' off the bolt/stud...
Specifically they are called
prevailing torque nuts... They are made in many different configurations... Some are staked on the side like the one that you show, some are staked on the top of the nut near the hole, and some use nylon inserts on the top of the nut to create drag on the shaft of the bolt and keep it from vibrating loose...
