Originally Posted by marcdelpercio

To me, a variation is any change in the text or artwork DESIGN of a card. This would not include such things as stray print marks, variances in color shading due to low or missing ink, or other variances due simply to inconsistencies in the printing process. Rather, this would include cards that have a different actual design, meaning a variation in text (missing/different letters, punctuation, stats, etc), or artwork (different team, insignia on uniform, border design, etc).

I believe that the advertising back is irrelevant as this was meant to be used interchangeably among all copies of the card. So, in my opinion, even cards that appear with only one ad back (T206 Demmitt & O'Hara St. Louis, T205 Mathewson "37-1" & Doc White "no quotes") should still be considered variations as the DESIGN of the card is different.