Whoa! I was almost nineteen when I saw Bonnie and Clyde in August, 1967. Like many of my generation, I was stunned by the film. I was just going into NYU's film school and it was one of the most talked-about films of the day. EVERYONE recognized it as a major change in the direction of American film. I saw the film many, many, many times... I am not telling you something I've deduced or gathered, I am telling you what is the case: Bonnie and Clyde was never a widescreen film, period. There was a light matte that was sometimes used and sometimes not, that served the purpose of very slightly taking off the top and bottom, that served, for instance, to hide Faye Dunaway's underwear in the opening scene, but it was nothing that would take away from the essential squareness of the image.
Here, try this simple test. Watch the credits, which are very simple, alternating names with photos "from the period," each one matted with a white border and very precisely placed. There are some pictures that sit in "portrait" mode (emphasizing the vertical) and others that are "landscape" oriented. These latter look equally fine in the widescreen and fullscreen formats, But the "portrait" images, centered on the screen, are cut off -- the white matte at the edges -- top and bottom with the letterboxing. Look at it! Rather than formally centered in the center of square black screen, it is truncated, chopped -- mutilated, by the letterboxing. Do you really imagine this was the graphic plan? That the film was shot to include the fully matted photo, but with the intention of cutting it off?
Of course not. Instead of trying to explain away what's being said here, the point is to become AWARE, and press the issue.