All projections were not cropped the same. If you're thinking of the Senator screening, that was cropped considerably to hide platter scratches on the edges of the print. The film would not have been cropped that much on a 2.35:1 screen in '77.
Mike Verta could explain more about the nuances of projection aperture and cropping.
As an anecdote, I just saw a screening of Fantasia where basically the entire picture area was positioned within the semi-translucent screen border - during loud parts, I could sometimes see the edge of the soundtrack through the left border. I could also vaguely make out the edges of the sprocket holes through the right border, and a teeny sliver of the previous frame through the top border.