Actually, the only quality improvement I could find was to reduce the advertised coverage of the lenses in the early 1960's. For instance that famous/infamous 90/6.8 was at one time claimed to cover 100 degrees, and later only 80 degrees. Since there was no change to the lens, that would indicate that people wanted sharper lenses so they made them sharper by disinheriting the un-sharp portion of the image circle.
Lenses that did not need extended coverage were always great. My long gone Rolleiflex 2.8 E2 Xenotar (1957 vintage) was a sharp as one could ask for, but notice that when they went from the f/3.5 to the f/2.8 version of that lens they also went from 75mm to 80mm which indicates that the 75mm would not quite have sharp corners in that format. My opinion was that the Xenotar was a little bit more contrasty than the Planar.
What I am saying here is that I have never seen any quality control issues in Schneider lens if they were in "as new" condition. By "as new" I mean in the mechanical/optical condition they came from the factory in, not cosmetic condition. Remember, a lot of old lenses have been FIXED by IDIOTS.
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