The SCD Standard Catalog lists the Jackson proof, along with 11 other hand cut blank backed cards that were cut from a 66 card proof sheet. I think for a time SCD had a copy of one of the sheets in Lemke's office. All cards on the sheet were blank back and had no autos. The 12 listed in SCD ( not Denny) are unique in that the proof shows the players in a different uniform and different team from their issued card.
So I assume there were at least 54 other blank back cards with no autos on that sheet. I have not seen a scan of the sheet to know if Denny is on it. And do not know if all the cards in the set can be found with blank backs and no autos from multiple proof sheets.
I am not sure if this 77 proof strip further complicates things. I won this recently. It must be a proof sheet because the pictured Cardinal did not appear in the set. I do not know if these cards were among the 66 cards on the sheet that produced the 12 check listed in SCD.
Maybe someone else can add something
I know we still need to touch basis further on the Fleer Cardinal stickers issue issues