Recently discovered there are two variations of the 13th issue NOW Cards with the Sapphire, Gold and Platinum Circle from Empress Casino in Joliet, IL. Here's a scan of the Sapphire card so you can see what issue I'm talking about.

One set of cards has the reverse paragraph of text closer to the casino name and the "Joliet, IL" line with about a 1mm space; and further from the mailing address at the bottom, with approx. a 3mm space. These cards all appear to have a brown mag stripe and were used in 2006.

The second variation has the reverse paragraph of text centered with approx. equal 2mm spaces before and after the paragraph. These cards all appear to have a black mag stripe and were used in 2007 to 2009 from the dates I've found on cards so far.

I've split the two variations into 13a and 13b issues to make them easier to document in the guides. Additional scans of the cards were added to the ChipGuide and should be live in a day or two. The differences are very subtle but the mag stripe colors make it pretty easy to spot.