Thank you for the kind offer, but I think the ding is part of the history of the shell so I will leave it as is. I'm quite excited to find that it is actually consistent with a shell from Long's battery.
Most people consider the "history" of an object to be "what happened to it before me".
I sometimes agree with this view, and generally I just preserve objects so they don't deteriorate further, avoiding restoration or re-finishing.
But sometimes I take the alternative view... if something is carefully and sensitively restored or an incomplete object is completed, then I may take the view that the very next day that restoration is also a valid part of its "history". A good policy with firearms is to put a piece of paper out of sight e.g. in the butt-trap or barrel channel documenting what I have done and when.
Love to hear how other forummers approach the issue of restoration and conservation.
The past is not dead. In fact, it's not even past.