[TUHS] Unix "Multiplexed Files"

The subject of Communication Files on DTSS came up recently, and Doug linked to this wonderful note: https://www.cs.dartmouth.edu/~doug/DTSS/commfiles.pdf Over on the Multicians list, I raised a question about the history of the DTSS emulator on Multics in response to that, which sadly broke down into antagonism, the details of which aren't terribly interesting. But Barry Margolin suggested that the closest modern Unix analogue of Communication Files were pseudo-TTYs, that had generated a dustup here. Doug's note suggests that Plan 9's userspace filesystems, aided and abetted by mutable namespaces and 9P as a common sharing mechanism, were a closer analogy. But I wonder if multiplexed files were perhaps an earlier analogue; my cursory examination of mpx(2) shows some similarities to the description of the DTSS mechanism. But I confess that I know very little about these, other than that they seem to be an evolutionary dead end (they don't survive in any modern Unix that I'm aware of, at any rate). I don't see much about them in my archives; Paul Ruizendaal mentioned them tangentially in 2020 in relation to non-blocking IO: they are, apparently, due to Chessen? Does anyone have the story here? - Dan C.