The story seems to be they were made for JMB. After his project someone got a set of 18-gauge barrels made for his Parker to use up the shells.
http://parkerguns.org/forums/showthr...light=18-gauge
http://parkerguns.org/forums/showthr...light=18-gauge
The Keith Kearcher article on the gun and shells is in
The Double Gun Journal, Volume Four, Issue 3, pages 127 to 130.