I assume you are referring to the iron "S" on the face of the chimney. This was a remodeling of a 1920's Tudor and the original chimney had an iron stabilizing rod that anchored the chimney to the roof which originally sloped away. The rod went through the chimney and the "S" was a decorative way to terminate the rod and spread the load across the masonry. When we added the second floor the new wall took the place of the rod to stabilize the chimney. We kept the decorative "S" plate for authenticity but it is no longer functional. I don't know of a supplier but I'm sure a custom ornamental iron fabricator could replicate.