I've seen two Parkers who's internal tapering began 6-7 inches from the muzzle. Many orders for shotguns, in the early years of the 20th century, specified certain pattern percentages, with certain size shot and sometimes with specific loading data.
Could very well be that in order to achieve such specifications, a certain amount of hand honing black magic was employed to get there. That could mean that what appear to be tapered chokes are really honed out bores.