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Marc S.E104-III is significantly more difficult than E104-II and E104-I. Fully 20% of the "set" has been catalogued in the last decade, and there are still more uncatalogued examples out there.
That website (which is a great reference resources) only looks at a subset of the caramel cards, too. If you want to do a comprehensive list, there are some huge toughies that need to be considered:
E223 G&B chewing gum is exceptionally tough...examples tend to sell for thousands upon thousands of dollars, even in low grade.
E221 are very tough
E222's are also very tough. Exceedingly rare, there was a collective furor when a handful of examples found their way onto Ebay last year.
E224 Texas Tommy's are also very tough. There are a few set builders of this issue, but finding specific type examples can be quite a challenge.
E300 Plow's Candy are among the more beautiful and larger issues form the 1910s. These are also exceedingly tough to find, with most examples coming from a large near set break-up a few years back.