I have no idea what that part is. However, on other 123 models, I believe the black vacuum line is only a vent. If that is the case with yours, it doesn't seem that it's worth worrying about.
That part is in a similar location to the transmission vacuum control valve on US models of some 123's.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13