I've never heard that before about the 3.5E3 Xenotar? Do you know what the difference is? JohnW
Yes, first Zeiss gave the Planar an extra element, and then Schneider followed. The reason was to improve sharpness in the corners at the largest apertures. That's the answer I got from Zeiss.
The rumours about "built-in UV-filter" and "to make it cheaper to produce" aren't true. But the sixth element didn't make the lenses more expensive for Franke & Heidecke, according to the Rollei engineer Claus Prochnow.
If you don't make comparisions between the five and six element Planars and Xenotars shot at full aperture, I doubt you can see any difference.