Just discovered there are TWO variations of the 37th issue One Club card from Mandalay Bay with Circus Circus - Las Vegas on the front. On all of the original 37th issue cards with the different casino names on the front, the last line of text in the reverse paragraph ends under "discretion" above. On the second variation that last line ends between "at" and "the" (before the word "discretion") due to the use of a slightly smaller text font. So far I've only found this on the Circus Circus cards.
Here's scans of the common front with the original reverse followed by the new found reverse so you can see the difference.