Almost any sporting Single-Shot. Maybe it's a set trigger, Swiss butt, a longer barrel, a heavier barrel, usual sights, pistol grip or something. I'm not trying to minimize the worth of such features, just point out how many there seem to be. It isn't easy. That's because it takes time as a collector and observer.
Research is also a big help.
On second thought, make that an entire shaker. In light of that, any precise claims of rarity must be based upon a complete review and manual statistical analysis of the entire spectrum of Single-Shot records before they fell victim to the flames. This is unlikely in the extreme. And officials at the Cody Firearms Museum have no record or knowledge of any such break-down being performed. What's more, anyone who has spent much time with the written record books on any Winchester model knows that an accurate deciphering of each and every entry is virtually impossible.