As mentioned in the caption, the photo is from the book "Der Flugzeugschlepp von den Anfängen
bis heute" (Aircraft towing - From the beginnings to today). and there's really no explicit description.
But it is shown in the chapter, which deals with the combination of standard tow, using a long line
and the so-called "rigid tow, where the towed aircraft was position directly behind the towing aircraft.
I can imagine, that this "flying catwalk" (as another translation of "Brückenlaufsteg", which would be
bridge catwalk, but seems to make not more sense to me) was made up of three independent flying
wing elements, that could be coupled to a single three winged glider by shortening the tow lines between
them in flight. But this is just an idea, maybe someone has mor information.