I only know one person who is really interested in these, although I'm sure there are others. He is the administrator of the website
www.savagesingleshots.com and the author of a book on 220s/219s and the 101, Garnett Stancil.
There are several threads discussing these on that site. I've seen both roll-engraved guns and acid-etched. The latter seem more common. Most, not all, were originally shotguns.
Mike Armstrong aka Mesa