A couple of things worthy of note I think. One the American designation of Skeet is the same constriction as the English call ImpCyl. Two the American ImpCyl = the English quarter () choke.
Also, one question. Has anyone ever seen a factory barrel marked "Extra Full"?
In around 65 years of fooling with shotguns, I cannot recall ever seeing one so marked. I know that some at least of the Poly Chokes had an Extra Full setting & can't recall for sure if the Cutts had an extra full tube or not, but have never seen an original barrel marked other than Full, except of course the more open chokes.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra