 Perhaps the most notable return was the "porthole" in the 1956–1957 Ford Thunderbird.It was provided as an option to improve rear-quarter visibility with the removable hardtop in place... 

Opera windows began reappearing in the early 1970s in such vehicles as the 1972 Continental Mark IV, kicking off a brief copycat trend that often left the distinction between hybrid and rear windows that combined elements of traditional rear windows styling and true opera windows of the 1930s and 1950s.“...The hottest thing going was the "porthole" window in the rear side pillar - called "opera windows" that came in during the horse and buggy [era]...