Unusual, yes, but not improv. It was a line recommended in some obscure chess periodical or other, by Kopylev, or someone. Fischer, being surprised by the McCutcheon, decided to try it out, hoping to surprise the surpriser, but it didn't work out because it just wasn't that good a line. He'd have done better sticking with the main lines.

7 bxc3 ?

Thanks

There'd be no way to know Fischer was going to play it unless Fischer, in turn, had known that Petrosian was going to play the McCutcheon, which he probably didn't (Petrosian had never played it before). Probably Fischer, wary of falling into a novelty, wanted to get out of Petrosian's "book" as quickly as possible, and played something offbeat that he remembered seeing somewhere. Had Fischer given the variation any serious study before the game, I doubt he would have played it at all, as it allows Black to equalize fairly easily.

Here's the way this game is described in Tigran Petrosian: His Life and Games>:

<In round thirteen the French Defence brought Petrosian yet another victory - in his game with Fischer. For this meeting he had long ago prepared an old and (for the present day) very rare continuation - the so-called McCutcheon variation. Tigran had travelled to Stockholm with this variation, but there had been no sense unleashing his secret move there, so he stored it for a more serious moment. Petrosian was not only relying on the variation itself, but on its psychological effect, for he knew that Fischer did not orientate very well in unknown situations.

"Curacao 1962"

He gives the source of the novelty 8 Ba5 as Nikolay Antonovich Kopaev, a theoretician noted mainly for his endgame analysis.