The Sports Americana Price Guide (1979) does not have price distinctions for most of the common high series cards from 1966, although some cards have premiums applied (e.g., team cards, semi-stars, or stars).
The CCP guide from 1983 also does not appear to have price distinctions, although team cards, star cards (Roberts, McCovey, McLain, Williams, and Perry), and presumably semi-stars (Davis, Scott, Piersall, Howser, Skowron) do have a premium applied.
The Baseball Card Price Guide, April 88 issue shows a 2x multiplier for #524, 528, 544, 545, 547, 548, 551, 556, 561, 570, 576, 579, 591, and 593 in addition to premiums for stars and teams. Cards such as 535, 543, 554, 555, 558, 563, 566, 567, 574, 584, 589, 596, and 597 have a minor premium applied to the standard common card pricing, presumably because, they have Dodger, Yankee, Met, Red Sox, or Tiger players.
So, it appears that the SP idea may have germinated sometime between 1983-1988, but as mentioned in an earlier post by Toppcat, the cards apparently designated as SPs probably achieved that distinction because of poor collation or distribution issues rather than actual print quantity variations.
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