Interesting what WGB had to say relating to his 'muffler' experiences. I have but limited experiences and knowledge that I can relate here. However, I do know that original 6.9 mufflers were made of Aluminized mild steel in the interests of longevity, and I do seem to recall that original muffler systems probably cost in the region of at least two thousand dollars - plus fitting, of course. It makes the 126 mufflers look pretty cheap in comparison.
On Gleaming Beauty, I had to replace the original FIRST muffler about five years ago as it had disintegrated internally and rattled like 'whatever'. I am confident that it was original, so it lasted the best part of probably twenty eight years. I replaced it with a muffler (very flat and wide) from genuine 350SL - it cost $300-00 from an after market supplier, and at the same time fitted a 'new' used stainless steel rear muffler and flanged the complete system in the process. Regards Styria