I recently acquired a beat 'n stripped Klein frameset (thanks The Sprinter!) in trade for some work on his other road machine, and found an odd metal shield of sorts on the outside of the chainstays, and the inside of the lower section of the stays (below the brake bridge to the dropouts only).
First, obligatory photo of the frame:
Between this plate and the aluminum frame is a layer of bondo blending the two - a thin layer of bondo also caresses every aluminum joint around the main triangle, even though the actual joints are smooth as silk. Can't figure out the latter.
As for this mysterious metal - it won't respond to sandpaper, and it seems reasonably smooth to the touch as well, despite its appearance. Anyone have an insight into what this is? I have no clue at all.
P.S.: Bad pic, but it has through-the-dropout rear derailer cable routing: