rarest backs

[darwinbulldog]

Lately I've become more interested in rare back variations on otherwise common cards. "Otherwise common" spans a wide range, but in general I'm thinking of cards that carry an ACC designation and that one can generally find at least one example of on eBay at any given moment. I'd like to invite the type collectors and anyone else to edit my first stab at a listing of these rare backs with common fronts -- and, if so inclined, to estimate how many total examples of the rare back exist and/or what proportion of all the cards in the set have that particular back. Are there some that I should drop from this list? Some I should add (assuredly yes)? And what are the toughest E94 and T212 backs? Are any of the T212 back ads so rare that they should be listed here?

E94 (?)
E98 Old Put
M101-4/5 Everybody's, Holmes to Homes, Mall Theater
T206 Old Mill (blue)
"T206" Ty Cobb
T207 Red Cross
T212 (?)
T216 Virginia Extra
W502 Greiners
W514 Hendler's, Barker
W515 Fleer, Jersey Ice Cream
W551 Wool's American Maid Bread
W573 Cafe du Monde
W575 Johnson, Haffner's, Clark's, Gassler's