I don't think the black box has been removed from your car... my guess is that it was never there. Late model 300d's don't have the change over valve on top of the valve cover. My original understanding was that '85 federal and '84 - '85 california cars had the newer 'vacuum control unit' instead of the change over valve type of system. However, my '84 300TD federal doesn't have this and I'm pretty sure its original... maybe the wagons made the switch sooner than the sedans?

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1985 MBZ 300DT
1969 MBZ 220d
1984 MBZ 300TDT
1981 VW Vanagon