There should be an arrow pointing to a line with the word 'Kaltwasserstand' above it- this would be the cold water level in the expansion tank. On my cars this line is just above the seam between the upper and lower parts of the expansion tank.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)