If the glass surface has many many tiny, barely visible cleaning marks, this will add to the "glow" of bright objects in a dark background, will create a "halo" around them, will make them look sharp and blurred in the same time. Many old lenses have this, e.g. one of my Sonnars also does. I will try to look up an example to show the effect (visible even in colour!).
Remy, i still think that the paper you see most old photos on, is a major factor. Probably also the film and the chemicals/development. Otherwise, - don't you have some old uncoated glass on any of your cameras?
Honestly, when I shoot my pre-wwII rolleiflex with its uncoated Tessar, i do get lower contrast if there's strong contrast in the scene, but there's no other difference between the end results with the Rolleicord or the Yashica D i had before, and those are single coated lenses od the same design so i can compare. Moreover, there's also no big difference with the novar anastigmat on my folder, if i stop the lens down and don't look in the corners.