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DonPer the discussion above, most people consider the modern era of cards to begin in 1974 since the 1973 Topps was the last to be issued in series.
Personally, I'd start the modern era in 1971 since that was the first set to have real color photographs of actual game play. (the 1953 Bowman Reese tinted b&w photo is the only exception I know of).
I can see how 1981 might be considered. 1989 being the first UD set, which has modern card innovations might also be a good cutoff date.