To the question of "how many of these have you actually seen ungraded?" I would respond with "once the owner decides to sell, and goes to an auction house, it gets sent to grade by the auction house before we ever got to see it in its raw state." No auction house is going to sell a raw 52 Mantle unless it is destroyed and an obvious "authentic" grade. Nor would any person with half a brain buy one ungraded off ebay. So the cards we actually get to see, to wit: the ones that are for sale, are all graded.
I would guess that there are quite a few ungraded mantles out there.

How many of the remaining Mantles have great corners.....corners may be more 'in' 10 years from now versus centering..who knows.