Ok, I have NEVER seen one with a World Championship back, only the normal back. So, in my opinion that would make it more rare than the usual one, of which i have probably seen 15-20 on the internet and 2-3 in real life. My LGS even has one for sale. Only problem is, no one particularly cares for collecting world championship cards so the market would be small, exclusively error collectors and the sort. Did you notice if it was an extra card in the package, or did the blank replace a certain card in the list?If "WC" is World Championship, then yes, it has a WC back.
NOTE: I'll be adding pictures to my first post when my internet decides to let me.