The isolation bushing is made of black nylon and is mounted to both sides of the metal post (one perside). The back side bushing is held in place by the through-the-post fitting. The bushing on the inside
of the post is held in place by a dot of silicone placed by the installer. The purpose of the bushing is to prevent dissimilar metals from touching one another. When they do, in an environment where there is
an electrolyte present (like salt air), the two metals can electrochemically react to one another, causing corrosion, the most common example of which is rust, which can stain your posts. Keeping the metals
apart with this bushing can help prevent that corrosion.