Twice each week, on Tuesdays and Fridays, the game makes special items available to lower-level players. These items previously were part of a Special Offer and were available only to players at or above level 90. The Under Level 90 Special Offer uses lower level requirements than the standard Special Offer, and often uses lower prices, as well.
It seems that from level 50 until level 90 the Special Offer it is exactly the Vintage Offer for level 90+. So the real low level Special Offer that you see in the table below is Under Level 50.
Items in the Under Level 90 Special Offer, or U-90 SO, recur more frequently than standard Special Offer and Vintage Offer items. For example, some U-90 SO items repeat every month or every other month. In contrast, Vintage Offer items usually appear 9 or 10 months after their original Special Offer, and Vintage Offer items rarely repeat.