Bumper shocks were phased in starting in the 1974 model year. I use the term phased in since Mercedes played rather loose with the year on the titles back then. But then so did a lot of german brands.

But the bumper on the 1971, while a good bumper, is really nothing to pin a hitch to. I have a 109 bumper in my shop which has a steel brace running down the length of it. Without the brace the rear bumper is a bit on the thinnish side.