Black to move
This position might serve as a fingerprint for the game, although the position after 19.Rd3 is more specific to this particular struggle, as it was Karpov's novelty (according to Informant 18--the move appears in two other games in 1974).
In the diagram position, Black faces a difficult choice. 16.Nde2 was first played in 1973, but had been analyzed following the fourth match game of Geller -- Korchnoi Candidates Quarter Final, 1971 where 16.Bh6 had been played. Korchnoi himself was the first to mention the move in Informant, suggesting it in annotations to Trofimov -- Nesis 1972 (15/407). It was relatively new when Korchnoi faced it over the board. Informant 17/461 presents the move as a novelty (the volume covering the first half of 1974) even though it had been played in a correspondence game the previous year.
Korchnoi played 16...Qa5, which is now given in the Encyclopedia of Chess Openings as dubious with Karpov -- Korchnoi 1974 as the first of several reference games in the notes. The mainline (ECO B 78, line 13) is 16...Re8, which had been played twice by James Edward Tarjan in a tournament in Torremolinos, Spain. Perhaps Korchnoi was familiar with these games, both of which Tarjan lost, when he opted for 16...Qa5.
Debate in the Annotations
After 16...Re8, the annotations in Chess Informant become interesting.
In Martin Gonzalez -- Tarjan 1974, Black played the critical line after 17.e5. Botvinnik omits this line from his annotations to Karpov -- Korchnoi (Informant 18/433). Botvinnik gives 17.e5 dxe5 18.g5+/- as a note to the game. Enver Bukic annotated Martin Gonzalez -- Tarjan for Informant. There 17.e5 was met by 17...Ng4, which later became the critical line. In a note, Bukic gives 17...dxe5 18.g5+-. Thus, Botvinnik's annotation downgrades White's advantage in the position from decisive to substantial.
Neither Bukic nor Botvinnik comment on the merits of 17.e5. However, another game played by Tarjan appears in the next issue of Informant, this time a Black win. Dragoljub Minić annotated Informant 19/403, and gave the move 17.e5 an interesting (Tarjan's opponent, Planinic, played 17.Bh6) with the line 17.e5!? Nxg4 18.fxg4 Bxg4 19.exd6 evaluated as unclear.
Two years later, Milan Matulović followed Planinic's idea of playing e5 one move later against Tarjan, who seems to have a knack for getting into theoretical battles in this line of the Yugoslav Attack against the Sicilian Dragon. That game with Matulović's comments proceeded from the diagram position: 16...Re8!? 17.Bh6 Bh8 18.e5! Nxg4 (only move--18...dxe5 19.g5+-) 19.fxg4 Bxg4 20.exd6 (Informant 21/389). Matulović played an improvement suggested by Minic at move 24.
Tibor Florian carries the debate further in annotations to Bernei -- Schneider 1976 (Informant 22/482). Bernei played 17.e5, which Florian assesses as an error. In the notes, he presents Matulović's line, 17.Bh6 Bh8 18.e5! Also in Informant 22, Henrique Mecking presents a line following from 17.e5, but with no evaluation. Mecking's annotations appear in Tan -- Mecking 1976 (Informant 22/486), where 17.Bd4 was played.
Mecking presents without comment both Matulović's line (17.Bh6 Bh8 18.e5 Nxg4 19.fxg4 Bxg4 20.exd6 Qxd6 21.Qxd6 exd6 22.Rxd6 Bg7 23.Bxg7 Kxg7) and 17.e5 Nxg4 18.fxg4 Bxg4 19.exd6 Qxd6 20.Qe1 Qa6. Neither line is referenced, but the latter is from Martin Gonzalez -- Tarjan 1974. Is Mecking's lack of evaluation marks itself a comment concerning a lack of clarity concerning the merits of both lines?
Commenting on Beliavsky -- Filguth 1976 (Informant 23/439), Minic repeats his evaluation 17.e5!? That game continued as Matulović -- Tarjan until move 26. The debate in Informant concerning 17.e5 seems to have ended there, but ECO presents 17.Bh6 as the main line and gives several reference games following 17.e5?! in notes.
Other later moves also exhibit similar changes in evaluation from one issue to another. These games, and the debate concerning the merits of variations following 16...Re8 contribute to an understanding of what did not take place in Karpov -- Korchnoi 1974, second match game. This game, won by Karpov, was one of five decisive games in the twenty-four game match that chose the World Championship challenger to Bobby Fischer, and thus chose the new World Champion the year following the match.