This object is a steel punch, used in metalworking to make holes in steel sheets. Steel is a difficult material to drill through, so metal workers looking to make a hole will ‘punch’ through using an object like this one, with a bevelled (meaning sloped or tapering) end.
This particular punch has a square cross-section. On the other side of the sculpture another punch is visible, this time with a circular cross-section. These cross-sections would be the shape of the hole made through the metal sheet when punched. Punches were common tools in meal workshops. César could have found these in the furniture workshop he used as a studio, or discarded at the scrap heap due to some defect.