I have searched Google and there are lots of references to the existence of the Robbe hydraulic parts but no one showing them installed or saying how they work. According to multiple references on some German forums, it appears they are made by George Bruedern and can be found online here:
http://www.georgbruedern.de/index.ph..._33360019.html
I wonder if they are a copy of the infamous and mythical Uli Meinhardt rotary valve design. I say infamous because they are so highly rated on German forums and mythical because people that are not already "in" with him apparently get no response to email inquiries. I don't think I have ever seen a photo of a Meinhard valve though. I belive that Damitz also makes a rotary valve (as does now RC4wd) whereas Leimbach and Kampfer make a sliding spool valve design.