Sorry for the confusion! That's the last thing I wanted to do. I suspect that most of the questions have been answered, but just to live up to the reputation my children would promote of me...
1) Modified - shotguns come from the factory with either no choking, or tightening of the bore at the muzzle end, or some degree of choke, typically either skeet choke (the least amount, designed for the game of skeet shooting, which is not the same thing as trap shooting), improved cylinder (more than skeet, but less than the tighter ones, good for upland hunting of birds, rabbits), modified (tighter than Improved cylinder, kind of a middle of the road - do it all choke on the hunting seen) improved modified (a choke primarily seen on trap guns - shooting at longer ranges and needing smaller, fuller patterns of shot to hit the clay target as it sails away from you), full choke (used for trap shooting, helpful for squirrel hunting, and when lead shot was allowed it was a duck and goose hunting choke, designed for tighter patterns at longer ranges - a turkey choke as well) and last but not least (actually, the MOST choke) the extra-full, super-full, super duper pepper upper full which is a product of the turkey hunting revolution, designed to put a small cluster of a large number of shot pellets on a turkey's head at the greatest distance possible. Hope that helps...
2) few and far between - back in the day of the Winchester model 97, tighter factory chokes were the norm, and appropriately so. In the first place, the game of shooting skeet (closer range, sharper angles of target travel relative to the shooter, therefore benefited by larger diameter swarms of shot, and therefore more "open" chokes - ie. skeet and improved cylinder) began at the very tail end of model 97 production, so very few were purchased for that purpose. Add to that the state of the shotgun shell at that point, no shot cup, typically softer shot, therefore more shot deformation (scrubbing out of round as it passed down the barrel of the gun) tighter chokes were required for most hunting purposes. I doubt, therefore, that there are many 97's with improved cylinder chokes. Most I've seen that were factory original have been full choked. The Winchester model 12, a later model obviously than the model 97 was the darling of the early skeet shooters from what I understand, still, try to find one in improved cylinder, let alone skeet! Again, the changes and improvements in shotgun shells relative to hunting purposes has been incredible, but it has come since the 1960's, with "shot wraps" and the beginnings of shot cups, and has improved logrythmically since then. The last 15 years have been nothing short of revolutionary in every aspect of shotgunning. This leads me to...
3)Appropriate chokes for specific purposes. Today, it's hard to buy a new shotgun with a single choke barrel, that is, improved cylinder, modified, OR full. Today, most barrels are threaded internally for choke inserts, and you can purchase tubes to screw into your barrel in just about any constriction you want. Due to the nature of today's shotshells, and the availability of choke inserts, I feel that most people would be best served with an improved cylinder choke in their gun, especially for typical shotgun efforts. It (an improved cylinder choke) typically produces decent patterns with even coverage and adequate pellet numbers to dispatch small game very well within the distance most hunters I know should be shooting at it. This is a vast overstatement to be sure - what about turkeys and predator hunting, and what about trap shooting, and... Anyway, for short range shooting at reactive targets, I believe most shooters would be best served by a shotgun with an improved cylinder choke. That's all I was trying to say. I sure didn't mean to be confusing, or demeaning by any stretch!
By now I have beaten the horse to death to the point that it is hard to tell it was a horse, but hopefully that's helpful at some level
Jamie