Martin:
Those cards are definitively, absolutely, positively Exhibit cards. I wrote an article on this issue years ago for Vintage & Classic Baseball Collector and another one in Old Cardboard (#12, Summer 2007). In a nutshell: there was a lot of conjecture that it was not an Exhibit issue because the backs of some said "Not to be used in Exhibit machines." I ran an ad in VCBC back in 2000 asking for data on the set and I got a response from Elwood "Woody" Scharf, the man who took the first stab at detailed check-listing of this issue for The Trader Speaks in the 1970s. In our correspondence he sent me an article mentioning that another collector had informed him that he'd seen one with the Exhibit coupon on the back. Woody did not have an example or copy. Since then I have personally seen three other cards with the coupons on their backs, with the "not to be used in Exhibit machines" slogan right beside them. One of our members who does not publicize his holdings has one and I had the chance to go over it carefully. It is absolutely an Exhibit card. Here is another example from that mixed baseball-boxing 4 on 1 issue you referenced that was made using the same artwork, an issue that Woody alluded to it in his articles but could not confirm the existence of (probably because it is really tough to find):
Just for a fun aside, here is an example of a strip card made from the same artwork as the PC back Exhibits (note that it is not a cut down 4 on 1--those exist too--but is readily distinguished from the Exhibits due to the thinner smoother stock used. Also below is a strip of the boxers from the related issue:
