Buckmaster was managed by London-based agent/producer Tony Hall, who was friendly with Davis and introduced the two of them during one of Davis' UK visits (1969 I believe, must have been that visit when he played the London Jazz Fest. and the one (filmed, now lost) night at Ronnie Scotts).
I've often wondered if, before those OTC sessions, Buckmaster played Davis his 'Chitinous' session recorded for Deram (reissued a few years ago on CD by Vocalion, now sadly sold out/OOP). The last track in particular has some of the repetitive, bass-riff driven brooding that permeates 'On The Corner'. Chitinous was released in 1971 I think.
I remember seeing that Polanski 'Scottish Play' on film projector at school back in the day but of course never knew of any Buckmaster connection ! First and only time I've ever seen it.
You are right--November 1969.
Buckmaster's notes to On the Corner indicate at the time he played Miles a 30-minute piece of his called "Joint Effort," featuring guitar, cello and organ improv over a repetitive bass and drum riff. Miles quite liked it and said it was similar to some music he had just recorded, that music being Bitches Brew.