Someone (Randall Neff) from the Computer History Museum has scanned a lot of the documents from Knuth's personal collection; search for "Dijkstra" on this page: https://archive.computerhistory.org/resources/text/Knuth_Don...

The following document is labelled as the source code (the OS was called "THE"; see https://en.wikipedia.org/w/index.php?title=THE_multiprogramm...): https://archive.computerhistory.org/resources/text/Knuth_Don... (254 pages of assembly code), but there are also other related documents listed on the page.