One can view the premise of "truly rare" in absolute terms (12 cards, or less). Or, another can take the view of
"supply vs. demand"....in my opinion, these two cards are truly rare, since the demand for them is very high.
Most of the 515 (of 524) subjects in the T206 set are available in the 1000's of cards. But to date, only 8 cards
of this error card of "Slow Joe" have been confirmed.
And so far, only 14 cards of the red Cobb with the Ty Cobb back have been found.
TED Z