According to the Chassis and Body service manual, page 98.0-280/1, the paint used is part # 000 9986 67 33, MB combination varnish, DB164 (deep dark grey, mat. It says it is also the color used on the outer skin at the frame side members, spare wheel recess, trunk, etc. Used on models 107, 116 and 123. So if you look at the bottom of the car AND if it has the original paint you will see the color. OR, look at the spare wheel well, the inner parts of the trunk on the sides, etc...
Hope this helps...

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C