What to make of this?
Over the years, Post Cereal has produced many different sets of baseball cards. They used to actually come in a box of cereal, sometimes with a redemption coupon to get the whole set for a bunch of boxtops. In 2003, Post produced a 31 card set, which came in a little box. The cards were actually made by Upper Deck. I don't know how Post distributed them. I got three sets in repack boxes. They used to sell these boxes in Toys R Us, where packs of cards were glued (with extremely strong glue) to a cardboard box (with just sides) and then the box was put into another box which had windows on the side so you could see what packs you were going to get. There was also a special surprise in the interior which you couldn't see. I got a few weird baseball things in those boxes.
These Post cards were pretty nice and featured all the big stars.
I got this odd card in one of the sets. I have no idea of what its supposed to be. My guess is that the cards were printed on a sheet which held 32 cards but since the set only had 31 cards (what, they couldn't find one more player who was good enough?), the blank space had this rainbow thing. Since only one of the sets I got has this, I'm sure it wasn't supposed to get in the box. You can't tell from the scan but the rainbow has a solid white border.
The best part about it is that it has the Upper Deck holographic logo on the back, which, of course, is the only way to tell it's the back.