This series is available with four different style backs. There are white backs, tan backs, and Ludlow backs with both red and black printing. The white and tan backs seem equally plentiful, but the Ludlows are more rare. Run Tony Shells is generally more difficult to find than the other stickers, because it was replaced with a double print of Chicken Fat at some point.
Sugarmess Gum is the only sticker that was reprinted in a later series (the 14th). The two can be distinguished by the copyright information on the front of the sticker. The 14th series version is dated 1975. The 2nd series version has no date.
Many of the designs in this series debuted in the Wacky Ads set from 1969, although the artwork is better on the 2nd series stickers.