Oh, well in that case may advice is to sell it as is, especially with the higher mileage. What it will cost to fix probably won't get a return on investment for you. Just disclose the situation, new owner might just drive it to it's grave and not care.

Also, there is a third screw back in the corner where your rust is.

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1983 W123 300TD US spec Turbo engine, with Euro bumpers and manual climate control, and manual transmission.