I found a similar document in my '86 300E. It was underneath the passenger side carpet and sound deadining ( I think that is a word)
pad. It is adhered to the sheetmetal that goes from the floorboard to the rocker. I believe it is a factory build sheet checkoff form, as the car went down the assembly line during construction. I noticed there were initials/signatures of perhaps, inspectors or the workers themselves signing off of work completed. I discovered it when I was disconnecting my Lambda sensor wire connector which comes thru the transmission tunnel enclosed in a nice wire trough underneath all that carpeting material that I was refering to. Oh, and mine is also written in German. My Porsche had a similar sticker in the trunk, under the carpeting, and a duplicate inside the Warranty and Maintenance service booklet. They were all the codes the factory uses for options which were built into the car. So if your car came with it's original service history maybe it is in the book also. I hope this helps.