Prestige and the Volumetrica both have M cut out bottom bracket shells.
The Volumetrica will have some form of rear brake cable housing control brazed on or in.

An unmarked bike just indicates repaint without a visual review. Repaints happen for many reasons, some benign, some not.

Study Bob Hovey's site, it will help quite a bit, but it is not finished.
A serial number or any and all bottom bracket stampings will assist in a broad way.
One thing I have to say is that let the frame tell you the date, don't take the owner's word, even if he or she bought it new, and it was in the mid 70's that is 35 years ago. Most just do not have that good a memory.