I think the same wrapper was used for 70 and 71 as I've never seen another style; even the small 1969 Super wrapper used similar lettering.
The last digit of the code represents year of conception in a way, it was assigned once a set was green-lit for production. For non-sports sets it can sometimes lag behind the year of issue for sets released early in a calendar year but for sports sets it seems like it also works for year of issue as I believe that's how they had to account to the leagues and/or players unions. There's probably a couple of sets that don't follow this pattern as well, Topps being Topps.
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