That's wierd. Over here in Europe squares are much more scarce than round ones...

As a lot of guys mentioned the squares were for the high end ones and here basically 90% of W123s got the round ones.

I am not sure about which one is better, so I'd decide by the looks. Well, that's what I did with mine, anyway. I couldn't stand rounds - probably because they are so common over here - and switched them to squares, which look way better, I think.

So, I'd suggest getting squares, anyway, even if you could find parts for both, but up to you!

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previous:
1977 W123 200D - the slowest car I've ever driven, 500k+ miles when sold
1992 W124 250D - 257k miles when sold
2002 VW Polo 1.4 - 100k miles when sold
current:
1987 W124 300D Turbo - 320k miles