The answer to that question seems to change with the latest Barrett - Jackson auction. I think the most current recognized car is the 1971 Hemi - Cuda convertible. But that is subject to change.
I don't know. I don't watch the B-J auctions (aptly named) but I was under the understanding that most of the really high end cars like the Hemicuda verts were more apt to change hands between high dollar collectors in private dealings rather than rolling across the auction block.
But you're right. Seems to be the more the collectors get down to the nuts and bolts, so to speak, in research some little thing like the "last one built" or the "first one built" or the "only one with this color combination" speaks to what the collectors call rarity and desirability.
To this point, it seems like the '71 Hemicuda convertibles are the "rarity" of the day.