I don’t know much about PCS Unix, but I did come across many references to Newcastle Connection (and Unix United) when researching early networking and the various approaches to giving early Unix a networking API. I think there is no other set of surviving sources for this. Maybe Holger disagrees, but I would say that PCS Unix is best placed in the “Early networking” section. By the way, for those interested, here is a start to read up on Unix United: https://en.wikipedia.org/wiki/Newcastle_Connection To some extent, it is similar to the “RIDE” software developed at Bell Labs Naperville by Priscilla Lu and to S/F Unix developed by GWR Luderer at Murray Hill. As far as I know the sources for both of those have been lost to time, afaik.