After looking over Dan's list (thanks!) I wonder if the switch to plasmat type was due to the cost of manufacturing. A dagor requires 3 (usually) equally-sized elements that need to be ground to perfectly match for the glass/glass interfaces. I'll assume that at the time Schneider was doing the final adjustments by hand grinding. Which would be why the first focal length they switched to a plasmat was the largest, the 355, which by virtue of having the largest glass elements would be the hardest to grind to a perfect fit.
And once the plasmat 355 was out there and accepted by the industry, they made the decision to switch the rest of the line too, because plasmats were increasingly used in process lenses starting about then perhaps due to the work Schneider themselves did in perfecting the design for the needs of graphic arts and printing.
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