msycamore said:
It's the opposite. If the rounded corners of the 35mm frames are visible, the frames aren't cropped enough if original theatrical aspect ratio and presentation is the goal.

But those aren't the rounded corners of the 35mm frames. If there's any scene in the workprint that naturally has the rounded corners in the same place (because 35mm frames had to be used) and the missing picture information can't be "restored" by duplication or the use other sources, this clipping mask would make sense.