If the key has three positions, horizontal, vertical and a few degrees CCW, try turning the key fully CCW, hold it there and push in.

That's how my W114 280C and W116 450SEL work.

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