Over the years the Mastro/Legendary team has sold a number of "printers proofs" which are easily recognized by having square corners and no wax. Here is a link to the latest group they sold.
http://www.legendaryauctions.com/Lot...x?lotid=116950
Though none of the auctions show the backs, none of the descriptions ever mentioned the cards having blank backs. Many of these square cornered cards are drifting around the hobby and show up a few times a year on eBay and in auction houses. All of the ones I have seen have had normally printed backs. Also none of them have had stamped numbers or show the kind of damage the card above shows.
I do not disagree with Leon that people at Heritage have seen many more cards than I have, however I do follow Stahl Meyers more intently than most. I would be interested if Marty Pritchard or Levi Bleam would purchase this card.