I have a red '79 450SL, the underside of the trunk is a very dark shiny grey, also a '82 380SL, the underside of the trunk is also a very dark shiny grey. The color is so dark it's almost black on both trunks. I'm sure it's the original color. If you pull open the battery cover you should see the same color on the inside of the fender.

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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