What about the Rote Sau on 1971? Wasnt that co-operation between the companies?
Yes, but with an important caveat.
Mercedes-Benz (specifically Erich Waxenberger, chief of the MB Experimental Department at that time), did provide Aufrecht and Melcher with a few M100 6.3-liter engines on an "under the table" basis to support the development of the 6.8-liter version of the engine as used in the Rote Sau for Spa and other races at the end of the 1960s and beginning of the 1970s. This support from Waxenberger was not sanctioned by MB, and perhaps even they never knew about it until much later.
Erich Waxenberger told me about this personally when I stayed with him in 2002 at his winter home in Switzerland. I will never forget the look on his face when I did a Google search of his name and all of the information available even then. He'd never seen that before.
The supplying of these engines to AMG was instrumental to them getting started in racing. It was primarily done by Waxenberger because MB was not officially supporting any type of racing at that time, stemming from a board decision made in the wake of the racing disaster they had in the 1950s. By supplying these engines and other assistance "under the table," Waxenberger was living a little vicariously through Aufrecht and Melcher. He even was an actual driver (using an alias) of the Rote Sau for a couple of races.
So
@2phast's statement and document really pertains to "official" and company-sanctioned support and co-operation between the companies. Anything prior to that would have been done on an individual and/or ad-hoc basis.
Waxenberger was also known for "unofficially" providing MB support in the form of parts and assemblies to assist other non-MB racing efforts, using MB vehicles, such as with the rallying efforts in the 1970s with the C107 models, and so forth. Obviously later on, MB decided to begin factory support again for racing efforts, to an increasing degree.
