I have one of these, same dimensions and same number of rivets holding the reflective strips on. It came to me in the trunk of my European Delivery '74 240D. I believe it came with the car when it was delivered new in Germany. There is no dedicated place to clip the triangle into the trunk lid like later cars have, but it does fit nicely leaning against the spare wheel. Maybe this is what M-B supplied before they went to the clip-in design. Looks like the same manufacturer as the other triangles I see in the photos above (Dambach?). Mine still has a plastic cover and there is some printed information on it.
