I cannot prove it, but I do agree with Dan. The odds are great it is 2 different sets.
I have the 4 sets of the Nebraska Cornhusker football. (Red and White) 2 years 73 & 74
When makes me believe there are 2 set?
When looking for my huskers I run upon many College teams with it on E-Bay and other sites with two different backings.
I have met a dealer which only does oddball stuff, and he was in the mind set with all his dealings that most of the college playing card sets had two back colors (team colors). There was a lot of "canasta" card playing in college during the 70s.
I meet a guy (family) that was collecting all types of playing cards. Mainly cards from casinos and towns they visited. All the College sports playing cards that he had seen or purchased had two sets with different backs. Which he did pick up some.
Plus others
Side note I was warned about buying singles marked one year. The card may be the same card that is in another year. Like lot of the cards in the Heros playing cards sets as the copyright mark on the box is the only that is different on some sets.