Kay is right, those bumpers are anodised from new. From experience the fronts seem to be fairly scratch-resistant, but those back ones certainly aint! It might also be age-related, as those on my '68 145 (same rear bumper) are like yours I reckon, but my later 164E ('72) are quite good.
Perhaps anodising improved over the years: I wonder if it is a costly business getting them re-plated the professionally polished - its been a long time since I visited a platers.
P
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