Did they introduce this categorization in 1966, or develop it later. I ask because I have a book published by Informant (Max Euwe, From Steinitz to Fischer) that uses a different system. R = miscellaneous, E = king pawn opening, and D = queen pawn openings.
The oldest print edition of Informant on my shelves is CI 30 (1980), which also uses the scheme in From Steinitz to Fischer. I only have CI 31 in electronic edition, but CI 32, which I have in the print edition has the A-E ECO system. Either 31 or 32 introduced the current system of five volumes.
It would seem that R became A about 1980-1981, E became B and C, and D became D and E.
On the other hand, according to Wikipedia, volumes A through E of ECO were issued 1974 through 1979.
Perhaps two different versions of the ECO classification existed 1974 through 1980.
ECO Codes
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Nov 14, 2016
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(ECO) code. In 1966, Chess Informant categorized the chess openings into five broad areas ("A" through "E"), with each of those broken down into one hundred subcategories ("00" through "99"). The openings were published in five volumes of ECO, with volumes labeled "A" through "E". This is a list of chess openings by the ECO classification.