Well I have never seen one, and I have seen a ton of W126's.

My 82 SD did have some nice rust on the trunk lid though, its still running though, just saw it the other day.

I have continued to collect W126 literature and I have yet to see any mention of aluminum in the structure.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200