I've owned several early and late W126s.
I don't think there is any basic difference.

The trim is different, which COULD require different mounting holes.
If so, then the fenders would have different part numbers.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles