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Have a look on herehttps://www.sl113.org/forums/index.php?topic=34367.msg251319#msg251319and herehttps://www.sl113.org/forums/index.php?topic=33164.msg241881#msg241881
I have an original 64 230sl and here is a photo. The central single drainage hole looks to be about 3-4mm in diameter.