RS1600 that are some very interesting photos you have posted again. And a very interesting ad.
If you read through the gt-eins thread on the zak escort, someone wrote that the Ringshausen car was used in the 1976 1000km at the nürburgring by Zakspeed as a genuine Gp5 car. I believe a pic was posted as well.
If you take a look at this:
http://www.teamdan.com/archive/wsc/1976/76nurb1000.html According to these results this is correct and driven by Klaus Ludwig (D)
Peter Hennige (D)
Siegfried Müller (D).
Also note the Zakspeed entered car of Finotto and Grano, (where the hell did that end up?)
However I am confused by the ringshausen ad as it says the escort has an engine tuned by Schrick, but Zakspeed livery is all over the car (if you take a closer look). I know Dr. Schrick did engine work for the Zak Capri Turbo cars. Perhaps he overhauled the customer Zakspeed engines?