I think you meant to write...

that the watch dial on the RIGHT had the mottling on it.

Yes, interesting and I think that if you could know the history of each watch, you'd find that the one on the left was infrequently worn, while the one on the right with the dial problem was worn daily. Daily wear can expose a watch to a lot of perspiration which has an uncanny way of working its way into a watch case over the years and corroding the metal parts therein. Brass dials seem to particularly prone to this effect. Sweat is high in ammonia and this, in vapor form, is known to have a corrosive effect on certain types of brass.

technoguy