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ZinnT231 is not a "mainstream" set and ought be discounted (sorry Leon) when answering a question of this nature. I think Marc is right on with his assessment of scarce "T" issues.
Darby's and G&B likewise are not mainstream. While admittedly rare they do not fall into a category of widely collected. Most collectors know of the rarity and owning one is for most out of reach and even thinking about collecting a set is impossible. E107 on the otherhand is the ultimate collectible "E" issue. Everyone(except the 25 Board Members who don't like caramel cards) wants one. Four or five collectors aspire to completing the set and are close. For me it is No. 1.