The "ding" was caused by a side pull brake caliper banging into the down tube. Or, a result of multiple bangs, over the years. I might add that this is a very common malady for high end vintage road bicycle frames.





A good quick fix that works and looks OK is to simply glue a piece of felt onto the offending spot. You can buy these little felt pads at dept stores or the like.

The repair (disguise) looked just fine on this Sekine SHX270, and helps to prevent further damage from occurring.