Yesterday, I purchased a Ruger Blackhawk .45 with two cylinders. It was advertised as "as new" in the box. It certainly looks new. A serial number check indicates it was made in 1986. From the exterior view, I can not find any differences in the cylinders. They appear identical. What would be the best way to semi-permanently mark the cylinders without damaging the finish? It would be nice to know at a glance what cylinder is installed. The gun is blued - not stainless. Any help would be appreciated.