Actually I wanted to Sitzverkleinerer not use and have in addition the roba 1850 E36 - ordered Sitzverkleinerer Prince Luis, but the fit not at all on the Roba Sit Up III, neither the material nor the form. Now I'm quite happy with the bundled version. In addition, this seat reducer is extremely easy care forgives carrot mash-splash: simply wipe with a damp cloth over it and the stain is gone :-)
The construction of the stairway highchair went by the way quite simple using the supplied wrench. It is at least not annoying than with IKEA furniture ...