Comment by egberts1
X Window Release 3 (X11R3) was introduced on Cray into UNICOS (a UNIX variant of Cray OS, COS) in late 1989 using ported 64-bit Xlib. But it was not widely used within small Cray community.
But MIT cooked up X11 "PROTOCOL" of Xlib in late 1985 to 1986 on Univac and Unix in C with many other X libraries written in Common Lisp.
X10R3 mostly stabilized the Xlib around a few platforms and CPU architecture (DDX) in a"long" preparation for X11R1 in September 1987.