Posted By:
Rob D.Leon,
I was sort of following the logic of this thread until you agreed that the Butter Cream set is one that likely can't be completed. Yet just a month or so ago a Ruth was offered, and even though its hammer price reached six figures, it's not like it was "impossible" to obtain. Like you said, there is more than one known. And though the other 29 cards are scarce, they're available if one is persistent.
I would think that sets that have multiple cards of which there are only 1-2 known would fit your descripton better, but again, I could be missing the intent of the thread. (Actually, that's very likely. 
)