Could it be for the heater pipe, to keep it in place, at the back of the head? I only have experience with the M103 and a similar bolt is used for that purpose.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).