Complex sentence: In 1940 he moved into a first-floor flat at 129 Beaufort Street, a bed-sitting room that he occupied until he emigrated to the United States in 1981.
Split into simpler sentences: In 1940 he moved into the bed-sitting room he would occupy for over four decades, the first floor apartment at 129 Beaufort Street. Here he stayed until he emigrated to the United States in 1981.