This was not a source I knew about before: Daniel Defert, “Lettre à Claude Lanzmann”, Les Temps Modernes, No 531-533, Vol 2, 1990, pp. 1201-1206.
It’s a short piece in a massive 1400 page collection devoted to Sartre, ten years after his death. Defert was asked for a contribution but replied with a long letter, which he says can be used however the recipient wants. They decided to publish it.
As well as discussing the intellectual disagreements between the two around the publication of Foucault’s Les mots et les choses [The Order of Things], and extending to The Archaeology of Knowledge, Defert says a bit about their overlapping political commitments in the 1970s. I was looking at it because it has a minor point about a 1950s text I’m currently working on, but it would have been a useful source when I was working on Foucault: The Birth of Power. It has barely ever been cited, as far as I can tell, so I thought others might find this useful.