The round style Euro lights, from what I have read in earlier threads on the subject, were the type installed on the diesel W123 models while the rectangular type were installed on the gas powered models. I installed the rectangular type on my '85 300D and they are sensitive to any vibration caused by the engine. I would imagine the round style are designed to handle the diesel vibrations a bit better than the rectangular ones.
I honestly have never liked the round style. I remember when I saw the first W123's introduced in 1977 with the round lights, I thought they looked hideous. That was the first thing I changed on my car after I got it.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012