Don't pay $30 to a dealership. Many locksmiths don't have any idea when they look at your key compared to the books- because our cars are old enough that they're not in the books. My local locksmith kept the old books around, was able to make a copy for me. I have an 84 300D. The key blank that you need is "X83" or "MB39". Mine is an Ilco brass key. I know a lot of you want steel keys, but I'd rather have a bunch of $3 brass keys get worn down than the brass insides of my lock getting worn down by the steel key. Hope this works for you, and good luck.

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1984 300D Turbo Diesel - 211700+ mileage (now sold)
http://www.maj.com/gallery/Ned/benz/sig.jpg
In any of my posts, it is safe to assume
that I'm talking about this car.