The Kelleher Auction listings have an annoying habit of saying "only 50 issued". It should say only 50 found, as the press sheet had 200 stamps (the other 150 have not been found).
All the black missing error stamps are very off-center toward the left and many are also moderately off-center toward the bottom, such as the one shown in the Kelleher Auction. There are copies with both horizontal and vertical centering slightly better, but not by much.
You can see examples of slightly better centering in a 2007 Shreves Auction
http://stampauctionnetwork.com/f/95/853.jpg and also the Datz error catalog. The stamp in the 2007 Shreves Auction sold for $425 (not including commission).
I think you will find most modern US errors don't even come close to selling for the Scott catalog values, unless a dealer has the only find. But once the dealer starts selling off the supply, the competition comes in and the actual future selling prices tend to be a hefty drop-off from the Scott catalog prices -- although the drop-off is not as much for errors on commemorative stamps because those errors tend to have lower quantities found.