Try using 'Apollo' as a starting point.
They started shipping windowed workstations on or
before 1981.
E.G.
http://www.zepa.net/apollo/
http://www.citi.umich.edu/apollo-archive/photo-gallery/dm.gif
also
the Window concept is mentioned repeatedly in
"Fundamentals of Interactive Computer Graphics"
Foley, VanDam (c)1982
if not
Principles of Interactive Computer Graphics"
Newman, Sproull (c)1979
"Mathematical Elements for Computer Graphics"
Rodgers, Adams (c)1976, 1 reference
John A.