<p>Back in the 60s and 70s, at least in the Northeast, at higher toned institutions, it was “Mr.” We would call them “Mr,” and they would call us “Mr.” (or Miss or Mrs). “Professor” was ok but much less common, asst and assoc professor would never be called with the modifier, and rarely as “professor.” “Dr,” except for an MD, was, and still is to my ear, a little vulgar, and of course a PhD was never just “Doctor” but “Doctor Smith.”</p>