The Preview Set (PC1 to PC6) was sold separately from the packs of random cards, and enabled collectors to see the range of what different sort of sets were available to find. It is sort of like getting to see a peak of what is behind door number three, in the hopes that you'd be enticed enough to keep on playing and gamble all your current winnings. Saying that though, each card was limited to 500, so they are fairly rare and rightly sought after in their own right.