Hello,
both the 1:3,5/75 mm Xenotar and Planar originally had 5 lenses. During the production run of the 3,5 F and with the growing use of colour films it became difficult for Zeiss and Schneider to fulfil the high quality standards of Rollei. So Zeiss changed I think in 1960 or 1961 the Planar from 5 to 6 lenses without official publicity. Schneider did the same with the Xenotar. All 1:2,8/80 mm Planars and Xenotars always had 5 lenses. The 1:3,5/75 mm Tessar of the Rolleiflex T was an improved and recomputed version of the original Tessar. Compared to the Planar and Xenotar the Tessar/Xenar has a certain residual amount of curvature of field, they need f/8 to f/11 for best sharpness until the corners. Stopped down it will be difficult to see a difference to a Planar/Xenotar. Cameras with Schneider lenses were offered a little bit cheaper and they had not the "reputation" and "glory" of the famous name Zeiss but inofficially there was the rumour that they were at least equivalent if not a little bit better than the Zeiss lenses. Even today we have to pay more for a Rolleiflex with a Zeiss lens than for one with a Schneider lens.