I am also interested in the early development of networked GUI's that
led to the X Window System. According to Wikipedia there was a W
Window System that predated X.
W which was done at Stanford and is definitely related to (direct parent of) X Windows. Cheriton of course is the place to start, although I've lost track of him. He's in the valley somewhere, he made billions funding Google ;-)
V Kernel and the Stanford University Network (SUN) terminals were all developed around the same time as W.
FWIW: Gettys & Schiefler might have of the W sources squirreled away - as they started with them at MIT; but rejected it as the base for a set of reasons which I forget now; but did take a number of the ideas. Jim showed me a demo of early X and W side-by-side a long time ago. IIRC the basic site protocol and the approach where the 'server' was on the local system and the 'client' was remote came from W. [I'll try to ask Gettys what he has off-line].