Some early information on DP's and SP's were derived from "tabulation data" methods, which was literally opening up a vending box and seeing what patterns emerged, which was often skewed as Topps distribution was horrible. Other data was determined when only the 132 card half sheets were known, i.e. before the 264 full press sheets had been identified in the hobby.
Between eBay and PSA pop counts (to a degree) DP's and SP's are pretty much established these days from the 1957-72 sets issued in series.
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