I see your point of view Bruno, and think it should be listed as a 1992 set or at least a 91-92 set. I don't remember Ballstrett very well but did they have cards in EVERY issue or just end of year ones? Did this happen in other years too or just this one. These are all questions that should be considered and explained in detail. It seems the replies have all been hung up on one of the problems or the other but not both problems together. It seems that both the copyright date AND the name of the set should be addressed together, not individually. Furthermore, even if the 1991 is in the name, that doesn't mean that alone makes it a 1991 set. I would nominate you to ask Admin for clarification on this set, but my vote would be make it a 1992 set with 1991 in the name. Even with a search feature, it shouldn't be too hard to find.
I would imagine another option would be asking Spaz why he named the set what he did when he created the listing. I'm not blaming him for all this, not by a longshot, I would imagine it was because of the 1991 in the name, but like other listings on this site, sometimes information comes to light that makes a set needing to be changed.
Or maybe we're both wrong and Doc and Marty McFly went a year into the future, took pictures and created cards, then got them copyrighted, then went back to 1991 and released them. GREAT SCOTT! CARD'S AS FAR AS THE EYE COULD SEE!
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In the process of updating my collection so don't trust any of my lists right now.