When it comes to garbage mattes, I think the safest assumption is that they were invisible, or very nearly so, on the equipment the film-makers themselves were using. They may have shown up anyway on some projectors, most probably associated with luminance and contrast errors in the screening conditions or other technical reasons, but were not intended to be seen and under ideal conditions should intrude minimally on the visible image. Therefore dialing them out into the black is the way to go in a proper digital restoration.