There have been many slight variances over the years in the card stock used by Topps. Some issues have very noticeable differences in the card stock used (IE 56 Topps gb/wb, 59 gb/wb, 66 6th ser, some 68s, & 69s). The bright white stock (versus the dull stock) used on the 66 Topps 6th series cards (and the 66 Topps FB set) to me appears to be a slightly lighter weight card stock than it's duller looking counterpart. With the 63 Topps set, I have seen some backs from the lower series that have a goldenrod colored back, very similar to the 68 MBs....I have always just considered the slight difference in the backs of the 63s due to Topps using a slightly different card stock.
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