Okay, so how did Lang do it? (spoilers)
I replayed that last "armchair" scene over and over and could not find any obvious cut-and-match edit, so I have to only conclude that it was one continuous shot and when the camera pulled in tight on Robinson, the stage crew changed the setting all around before the camera pulled back out.
There just wasn't the CGI and green screen technology back then to fake it and clean things up so I figured it must have been done the old-fashioned way - through great planning and skill.
Any other thoughts?