1952 Bowman - Blank Bank or Proof?

[thatkidfromjerrymaguire]

In my ongoing quest to put together a nice mid-grade 1952 Bowman set, I thought it would be fun to add a Proof to my collection. I recently purchased a 1952 Bowman Billy Johnson "Blank Back" card pictured here:
blank.JPG

Now obviously that is not authenticated as a PROOF, but only an authentic "Blank Back". So my question is, are all Blank Back 1952 Bowman cards technically a Proof? If NOT, then what are they, just a print defect?

The seller of the card (who was a good, reputable seller, with fast shipping whom I would do business with at any time) conjectured that SGC may now be labeling all proofs now only as "Blank Back" with a grade of "Authentic" because proofs are typically cut from a larger sheet...hence the "Authentic" label. The same seller also had another 1952 Bowman with a blank back in an OLD SGC holder that identified the card as a PROOF, and gave a numerical grade. It was the seller's opinion that the two cards had identical characteristics, which led him to believe that SGC now authenticates blank back cards only as "Blank Back" Authentic cards instead of Proofs with a grade. Does anyone have any insight in to this, and whether my Blank Back card is an actual proof?

For reference, here is a picture of the other card in the old SGC holder labeled Proof. I do not own this card, but am showing for reference. Thanks to anyone who might have information to share!

proof.JPG