Found an old article on the Tele-Xenar 360 in which they imply that the rear cluster could be interchanged with others to make longer lenses, and there was a mention elsewhere of a supplemetary attachment for the existing rear element to make it even longer. But gosh, it was only a mention.
This is hardly conclusive, but there's no mention of TeleXenars with interchangeable rear cells in the Vade Mecum or of supplementary negative lenses to go on the rear.
In a leaflet from 1938 (which is available on Schneider's website) there is mention of using supplementary lenses on the rear element, but it references another page in the doc that isn't up on the website
I have a 360 mm Tele-Xenar (about 50 years old), it is in what is almost certainly the original shutter and has only one aperture scale. Have seen other examples that were the same, no aperture markings for convertible use.