Slotted Lock Nut
This striking object is a type of steel nut known as a slotted lock nut, or just slotted nut. Once a nut like this one is threaded onto a bolt a pin can be passed from one of the six slots on the nut, through a hole in the bolt shaft, to the corresponding slot on the other side of the nut. This means that the nut can no longer rotate around the bolt, and therefore cannot be loosened by vibrations.
This sort of nut is commonly used in vehicle manufacture. The scrap yard where César collected the junk for this sculpture dealt with unwanted cars – taking them apart and preparing the metal for recycling. It is not surprising that in this yard César would find an object such as this slotted nut.