"This is obviously the biggest single jump in the entire set of photographs, and the place where I most wish that Bill had dated them. It's tempting to say that the change was caused by the famous Xerox PARC visit, which took place in mid-December 1979, but Bill thinks that the windows predated that, although he can't say for sure."
"Although a single unit sold for $16,000, a typical office would have to purchase at least 2 or 3 machines along with a file server and a print server. Dropping $50,000 to $100,000 for a complete installation was not an easy sell."
http://en.wikipedia.org/wiki/Xerox_Star
Also Folklore.Org says the same thing: http://folklore.org/StoryView.py?project=Macintosh&story...
"This is obviously the biggest single jump in the entire set of photographs, and the place where I most wish that Bill had dated them. It's tempting to say that the change was caused by the famous Xerox PARC visit, which took place in mid-December 1979, but Bill thinks that the windows predated that, although he can't say for sure."