When: 1972-1977 Why: First truly folding SLR camera, First instant-film SLR camera.
Apparently Polaroid had to create an SLR camera for their instant film. Luckily they made it really portable, and it became somewhat popular. There were many variations later (I was able to find at least 28 different models). There were also non-folding models of SX-70 and numerous international versions which were similar to the USA models. SX-70 model lines were manufactured until 1986 (1982 by some sources), and the compatible film was discontinued in 2006.