Got to check one of these amazing cards out in hand, Thanks Andy. It is my belief that they are printed on the same card stock as the All-Star Game Commemorative Set.
http://www.ebay.com/itm/Ken-Griffey-...3AX&rmvSB=true I compared the card stock and thickness to every 1991 Topps product I have including all the store brands they print and it was the only one that was the same exact thickness.