I have a 1985 300TD and a 1990 300D. The keys are different. The 85 is cut like a normal, 2-sided key, but the 90 is cut along the sides with a solid segment in the middle. The 85 key will fit into the 90 slot, but the 90 key will not fit in the 85 slot.

So if my cars are typical, it could fit a w123 or a w124, but not both. The key blanks are different for these, at least for the Diesels.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)