That looks like a handy-dandy item...where does it come from?
Well everyone has chimed in. This tool in itself does not make sure the sights are correct. It does make sure the scope is perfectly centered vertically over the barrel. The rest of the procedure is using the plumb bob ect. Read thishttps://www.pyramydair.com/blog/2014/05/how-to-level-a-scope/This device allows you to make sure the vertical line of the reticule bisects the center of the barrel.