This is a 1lb plumb bob, manufactured from steel and then zinc plated. This plumb bob also has holes that allow the plumb bob to be threaded onto a string.

Plumb bobs are incredibly useful tools within the drainage industry, as they are used as a guide to identify whether something has been installed vertically, or plumb. The plumb bob simply needs to be attached to a string, and then held up, and gravity will cause the plumb bob to create a vertical line. 

Plumb bobs can also be used to mark a point that is directly above or below an existing point, for example from ceilings to floors and vice versa.