I don't recall the source (it will come to me after signing off)but I read somewhere that "Improved Cylinder" was the term used by English gunmakers to describe a boring technique. Apparently some makers, to maintain their position that choke boring was useless but to satisfy customers asking for closer shooting guns, used a method that constricted the bore only slightly and very gradually over a longer section, so as to be nearly indistinguishable from true cylinder in appearance. Thus the term "improved" cylinder. Eventually chokes became the general rule but the term stuck around to describe a choke of nominal constriction.
Indeed, I have an English 12b, the right barrel of which is not stamped "choke" but has a nearly continuous reduction in bore size beginning about 15" inches from the muzzle, to a final constriction of about 0.005
