Does it look like something you could file out of a piece of key stock? The boy's 300SD is beginning to have issues with the key not turning and if it's just that little part, I'd happily sit at a vise with a bunch of jeweler's files to make a new one.

Brass key stock is available at McMaster.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120