This is a rear seatbelt. I think it came from a 1979 300SD. It will fit either driver or passenger side, and should fit all years of W116 and W123, however is the 1975-1980 style. It seems functional, and the webbing looks good. If you want to clean it up, I like to use a gentle cleaner like diluted Simple Green to clean the webbing, and then apply 303 Aerospace Protectant. Normal rust/pitting/corrosion for the age. No mounting bolts are included. What you see is what you get, no guarantees. $15 plus shipping.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles