Ted Schefelbein will know the name of the choke; but for now there is some info from an article by Roger Barlow. "Even a true cylinder throws too tight a pattern...at 12 to 20 yards....
...some French gunmakers offer a cylinder barrel having very light rifling of slow twist which produces a usefully broad pattern with ordinary shotshells...effective in the 12 to 22 yard range....In effect it makes a superior "spreader" load of any normal shotshell...."