I'm hoping that someone can recommend a system for hanging paintings in a home, where oils in somewhat heavy frames are changed frequently, requiried heights in the same location therefore vary, and where the homeowner wants as "invisible" a system as possible.  Therefore, the old fashioned single brass rod with the screw adjustment will not do.  Nor does she want the ceiling molding with the brass S-clips with dual wires hanging down to each painting.
 
Does anyone know of a source of a good "invisible" or "semi'visible" system for a frequently rotated collection?   /many thanks / ken brown