Hey Fred!
I am wondering if they just classified both the player sets and the Sports Challenge discs as "1971 Mattel Instant Replay" without distinguishing them. Seems as though PSA's heart may not have been into this set, since they stopped grading it!
From your list, I suspect the Clemente, Koufax and Maris in the baseball section, and the Bill Russell in the basketball are all really from the SC series. I think I saw a listing on eBay for the Russell several years ago and if I recall correctly it was a double sided Sports Challenge disc.
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