Olberman has some SCD articles on the 68 and 69 sets in which he notes Topps was using old photos until the 5th series or so in 69 due to disputes with the rising Marvin Miller and the Player's Association seeking bigger licensing fees. I guess timing and production numbers may have been affected, and after agreement was reached maybe Topps went to town with new photos in the later series cards in 69. Old photos in 68 may have led to earlier production of late series cards in greater numbers ? Here is one copy of the Olberman articles
http://www.sportscollectorsdigest.com/1969-topps/